Port `multi_buffer` to gpui2 (#3193)

Julia created

Release Notes:

- N/A

Change summary

Cargo.lock                                |   47 
Cargo.toml                                |    1 
crates/gpui2/src/subscription.rs          |    2 
crates/multi_buffer2/Cargo.toml           |   78 
crates/multi_buffer2/src/anchor.rs        |  138 
crates/multi_buffer2/src/multi_buffer2.rs | 5393 +++++++++++++++++++++++++
6 files changed, 5,658 insertions(+), 1 deletion(-)

Detailed changes

Cargo.lock 🔗

@@ -5035,6 +5035,53 @@ dependencies = [
  "workspace",
 ]
 
+[[package]]
+name = "multi_buffer2"
+version = "0.1.0"
+dependencies = [
+ "aho-corasick",
+ "anyhow",
+ "client2",
+ "clock",
+ "collections",
+ "convert_case 0.6.0",
+ "copilot2",
+ "ctor",
+ "env_logger 0.9.3",
+ "futures 0.3.28",
+ "git",
+ "gpui2",
+ "indoc",
+ "itertools 0.10.5",
+ "language2",
+ "lazy_static",
+ "log",
+ "lsp2",
+ "ordered-float 2.10.0",
+ "parking_lot 0.11.2",
+ "postage",
+ "project2",
+ "pulldown-cmark",
+ "rand 0.8.5",
+ "rich_text",
+ "schemars",
+ "serde",
+ "serde_derive",
+ "settings2",
+ "smallvec",
+ "smol",
+ "snippet",
+ "sum_tree",
+ "text",
+ "theme2",
+ "tree-sitter",
+ "tree-sitter-html",
+ "tree-sitter-rust",
+ "tree-sitter-typescript",
+ "unindent",
+ "util",
+]
+
 [[package]]
 name = "multimap"
 version = "0.8.3"

Cargo.toml 🔗

@@ -61,6 +61,7 @@ members = [
     "crates/menu",
     "crates/menu2",
     "crates/multi_buffer",
+    "crates/multi_buffer2",
     "crates/node_runtime",
     "crates/notifications",
     "crates/outline",

crates/gpui2/src/subscription.rs 🔗

@@ -47,8 +47,8 @@ where
                     subscribers.remove(&subscriber_id);
                     if subscribers.is_empty() {
                         lock.subscribers.remove(&emitter_key);
-                        return;
                     }
+                    return;
                 }
 
                 // We didn't manage to remove the subscription, which means it was dropped

crates/multi_buffer2/Cargo.toml 🔗

@@ -0,0 +1,78 @@
+[package]
+name = "multi_buffer2"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[lib]
+path = "src/multi_buffer2.rs"
+doctest = false
+
+[features]
+test-support = [
+    "copilot2/test-support",
+    "text/test-support",
+    "language2/test-support",
+    "gpui2/test-support",
+    "util/test-support",
+    "tree-sitter-rust",
+    "tree-sitter-typescript"
+]
+
+[dependencies]
+client2 = { path = "../client2" }
+clock = { path = "../clock" }
+collections = { path = "../collections" }
+git = { path = "../git" }
+gpui2 = { path = "../gpui2" }
+language2 = { path = "../language2" }
+lsp2 = { path = "../lsp2" }
+rich_text = { path = "../rich_text" }
+settings2 = { path = "../settings2" }
+snippet = { path = "../snippet" }
+sum_tree = { path = "../sum_tree" }
+text = { path = "../text" }
+theme2 = { path = "../theme2" }
+util = { path = "../util" }
+
+aho-corasick = "1.1"
+anyhow.workspace = true
+convert_case = "0.6.0"
+futures.workspace = true
+indoc = "1.0.4"
+itertools = "0.10"
+lazy_static.workspace = true
+log.workspace = true
+ordered-float.workspace = true
+parking_lot.workspace = true
+postage.workspace = true
+pulldown-cmark = { version = "0.9.2", default-features = false }
+rand.workspace = true
+schemars.workspace = true
+serde.workspace = true
+serde_derive.workspace = true
+smallvec.workspace = true
+smol.workspace = true
+
+tree-sitter-rust = { workspace = true, optional = true }
+tree-sitter-html = { workspace = true, optional = true }
+tree-sitter-typescript = { workspace = true, optional = true }
+
+[dev-dependencies]
+copilot2 = { path = "../copilot2", features = ["test-support"] }
+text = { path = "../text", features = ["test-support"] }
+language2 = { path = "../language2", features = ["test-support"] }
+lsp2 = { path = "../lsp2", features = ["test-support"] }
+gpui2 = { path = "../gpui2", features = ["test-support"] }
+util = { path = "../util", features = ["test-support"] }
+project2 = { path = "../project2", features = ["test-support"] }
+settings2 = { path = "../settings2", features = ["test-support"] }
+
+ctor.workspace = true
+env_logger.workspace = true
+rand.workspace = true
+unindent.workspace = true
+tree-sitter.workspace = true
+tree-sitter-rust.workspace = true
+tree-sitter-html.workspace = true
+tree-sitter-typescript.workspace = true

crates/multi_buffer2/src/anchor.rs 🔗

@@ -0,0 +1,138 @@
+use super::{ExcerptId, MultiBufferSnapshot, ToOffset, ToOffsetUtf16, ToPoint};
+use language2::{OffsetUtf16, Point, TextDimension};
+use std::{
+    cmp::Ordering,
+    ops::{Range, Sub},
+};
+use sum_tree::Bias;
+
+#[derive(Clone, Copy, Eq, PartialEq, Debug, Hash)]
+pub struct Anchor {
+    pub buffer_id: Option<u64>,
+    pub excerpt_id: ExcerptId,
+    pub text_anchor: text::Anchor,
+}
+
+impl Anchor {
+    pub fn min() -> Self {
+        Self {
+            buffer_id: None,
+            excerpt_id: ExcerptId::min(),
+            text_anchor: text::Anchor::MIN,
+        }
+    }
+
+    pub fn max() -> Self {
+        Self {
+            buffer_id: None,
+            excerpt_id: ExcerptId::max(),
+            text_anchor: text::Anchor::MAX,
+        }
+    }
+
+    pub fn cmp(&self, other: &Anchor, snapshot: &MultiBufferSnapshot) -> Ordering {
+        let excerpt_id_cmp = self.excerpt_id.cmp(&other.excerpt_id, snapshot);
+        if excerpt_id_cmp.is_eq() {
+            if self.excerpt_id == ExcerptId::min() || self.excerpt_id == ExcerptId::max() {
+                Ordering::Equal
+            } else if let Some(excerpt) = snapshot.excerpt(self.excerpt_id) {
+                self.text_anchor.cmp(&other.text_anchor, &excerpt.buffer)
+            } else {
+                Ordering::Equal
+            }
+        } else {
+            excerpt_id_cmp
+        }
+    }
+
+    pub fn bias(&self) -> Bias {
+        self.text_anchor.bias
+    }
+
+    pub fn bias_left(&self, snapshot: &MultiBufferSnapshot) -> Anchor {
+        if self.text_anchor.bias != Bias::Left {
+            if let Some(excerpt) = snapshot.excerpt(self.excerpt_id) {
+                return Self {
+                    buffer_id: self.buffer_id,
+                    excerpt_id: self.excerpt_id.clone(),
+                    text_anchor: self.text_anchor.bias_left(&excerpt.buffer),
+                };
+            }
+        }
+        self.clone()
+    }
+
+    pub fn bias_right(&self, snapshot: &MultiBufferSnapshot) -> Anchor {
+        if self.text_anchor.bias != Bias::Right {
+            if let Some(excerpt) = snapshot.excerpt(self.excerpt_id) {
+                return Self {
+                    buffer_id: self.buffer_id,
+                    excerpt_id: self.excerpt_id.clone(),
+                    text_anchor: self.text_anchor.bias_right(&excerpt.buffer),
+                };
+            }
+        }
+        self.clone()
+    }
+
+    pub fn summary<D>(&self, snapshot: &MultiBufferSnapshot) -> D
+    where
+        D: TextDimension + Ord + Sub<D, Output = D>,
+    {
+        snapshot.summary_for_anchor(self)
+    }
+
+    pub fn is_valid(&self, snapshot: &MultiBufferSnapshot) -> bool {
+        if *self == Anchor::min() || *self == Anchor::max() {
+            true
+        } else if let Some(excerpt) = snapshot.excerpt(self.excerpt_id) {
+            excerpt.contains(self)
+                && (self.text_anchor == excerpt.range.context.start
+                    || self.text_anchor == excerpt.range.context.end
+                    || self.text_anchor.is_valid(&excerpt.buffer))
+        } else {
+            false
+        }
+    }
+}
+
+impl ToOffset for Anchor {
+    fn to_offset(&self, snapshot: &MultiBufferSnapshot) -> usize {
+        self.summary(snapshot)
+    }
+}
+
+impl ToOffsetUtf16 for Anchor {
+    fn to_offset_utf16(&self, snapshot: &MultiBufferSnapshot) -> OffsetUtf16 {
+        self.summary(snapshot)
+    }
+}
+
+impl ToPoint for Anchor {
+    fn to_point<'a>(&self, snapshot: &MultiBufferSnapshot) -> Point {
+        self.summary(snapshot)
+    }
+}
+
+pub trait AnchorRangeExt {
+    fn cmp(&self, b: &Range<Anchor>, buffer: &MultiBufferSnapshot) -> Ordering;
+    fn to_offset(&self, content: &MultiBufferSnapshot) -> Range<usize>;
+    fn to_point(&self, content: &MultiBufferSnapshot) -> Range<Point>;
+}
+
+impl AnchorRangeExt for Range<Anchor> {
+    fn cmp(&self, other: &Range<Anchor>, buffer: &MultiBufferSnapshot) -> Ordering {
+        match self.start.cmp(&other.start, buffer) {
+            Ordering::Equal => other.end.cmp(&self.end, buffer),
+            ord => ord,
+        }
+    }
+
+    fn to_offset(&self, content: &MultiBufferSnapshot) -> Range<usize> {
+        self.start.to_offset(content)..self.end.to_offset(content)
+    }
+
+    fn to_point(&self, content: &MultiBufferSnapshot) -> Range<Point> {
+        self.start.to_point(content)..self.end.to_point(content)
+    }
+}

crates/multi_buffer2/src/multi_buffer2.rs 🔗

@@ -0,0 +1,5393 @@
+mod anchor;
+
+pub use anchor::{Anchor, AnchorRangeExt};
+use anyhow::{anyhow, Result};
+use clock::ReplicaId;
+use collections::{BTreeMap, Bound, HashMap, HashSet};
+use futures::{channel::mpsc, SinkExt};
+use git::diff::DiffHunk;
+use gpui2::{AppContext, EventEmitter, Model, ModelContext};
+pub use language2::Completion;
+use language2::{
+    char_kind,
+    language_settings::{language_settings, LanguageSettings},
+    AutoindentMode, Buffer, BufferChunks, BufferSnapshot, CharKind, Chunk, CursorShape,
+    DiagnosticEntry, File, IndentSize, Language, LanguageScope, OffsetRangeExt, OffsetUtf16,
+    Outline, OutlineItem, Point, PointUtf16, Selection, TextDimension, ToOffset as _,
+    ToOffsetUtf16 as _, ToPoint as _, ToPointUtf16 as _, TransactionId, Unclipped,
+};
+use std::{
+    borrow::Cow,
+    cell::{Ref, RefCell},
+    cmp, fmt,
+    future::Future,
+    io,
+    iter::{self, FromIterator},
+    mem,
+    ops::{Range, RangeBounds, Sub},
+    str,
+    sync::Arc,
+    time::{Duration, Instant},
+};
+use sum_tree::{Bias, Cursor, SumTree};
+use text::{
+    locator::Locator,
+    subscription::{Subscription, Topic},
+    Edit, TextSummary,
+};
+use theme2::SyntaxTheme;
+use util::post_inc;
+
+#[cfg(any(test, feature = "test-support"))]
+use gpui2::Context;
+
+const NEWLINES: &[u8] = &[b'\n'; u8::MAX as usize];
+
+#[derive(Debug, Default, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
+pub struct ExcerptId(usize);
+
+pub struct MultiBuffer {
+    snapshot: RefCell<MultiBufferSnapshot>,
+    buffers: RefCell<HashMap<u64, BufferState>>,
+    next_excerpt_id: usize,
+    subscriptions: Topic,
+    singleton: bool,
+    replica_id: ReplicaId,
+    history: History,
+    title: Option<String>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum Event {
+    ExcerptsAdded {
+        buffer: Model<Buffer>,
+        predecessor: ExcerptId,
+        excerpts: Vec<(ExcerptId, ExcerptRange<language2::Anchor>)>,
+    },
+    ExcerptsRemoved {
+        ids: Vec<ExcerptId>,
+    },
+    ExcerptsEdited {
+        ids: Vec<ExcerptId>,
+    },
+    Edited {
+        sigleton_buffer_edited: bool,
+    },
+    TransactionUndone {
+        transaction_id: TransactionId,
+    },
+    Reloaded,
+    DiffBaseChanged,
+    LanguageChanged,
+    Reparsed,
+    Saved,
+    FileHandleChanged,
+    Closed,
+    DirtyChanged,
+    DiagnosticsUpdated,
+}
+
+#[derive(Clone)]
+struct History {
+    next_transaction_id: TransactionId,
+    undo_stack: Vec<Transaction>,
+    redo_stack: Vec<Transaction>,
+    transaction_depth: usize,
+    group_interval: Duration,
+}
+
+#[derive(Clone)]
+struct Transaction {
+    id: TransactionId,
+    buffer_transactions: HashMap<u64, text::TransactionId>,
+    first_edit_at: Instant,
+    last_edit_at: Instant,
+    suppress_grouping: bool,
+}
+
+pub trait ToOffset: 'static + fmt::Debug {
+    fn to_offset(&self, snapshot: &MultiBufferSnapshot) -> usize;
+}
+
+pub trait ToOffsetUtf16: 'static + fmt::Debug {
+    fn to_offset_utf16(&self, snapshot: &MultiBufferSnapshot) -> OffsetUtf16;
+}
+
+pub trait ToPoint: 'static + fmt::Debug {
+    fn to_point(&self, snapshot: &MultiBufferSnapshot) -> Point;
+}
+
+pub trait ToPointUtf16: 'static + fmt::Debug {
+    fn to_point_utf16(&self, snapshot: &MultiBufferSnapshot) -> PointUtf16;
+}
+
+struct BufferState {
+    buffer: Model<Buffer>,
+    last_version: clock::Global,
+    last_parse_count: usize,
+    last_selections_update_count: usize,
+    last_diagnostics_update_count: usize,
+    last_file_update_count: usize,
+    last_git_diff_update_count: usize,
+    excerpts: Vec<Locator>,
+    _subscriptions: [gpui2::Subscription; 2],
+}
+
+#[derive(Clone, Default)]
+pub struct MultiBufferSnapshot {
+    singleton: bool,
+    excerpts: SumTree<Excerpt>,
+    excerpt_ids: SumTree<ExcerptIdMapping>,
+    parse_count: usize,
+    diagnostics_update_count: usize,
+    trailing_excerpt_update_count: usize,
+    git_diff_update_count: usize,
+    edit_count: usize,
+    is_dirty: bool,
+    has_conflict: bool,
+}
+
+pub struct ExcerptBoundary {
+    pub id: ExcerptId,
+    pub row: u32,
+    pub buffer: BufferSnapshot,
+    pub range: ExcerptRange<text::Anchor>,
+    pub starts_new_buffer: bool,
+}
+
+#[derive(Clone)]
+struct Excerpt {
+    id: ExcerptId,
+    locator: Locator,
+    buffer_id: u64,
+    buffer: BufferSnapshot,
+    range: ExcerptRange<text::Anchor>,
+    max_buffer_row: u32,
+    text_summary: TextSummary,
+    has_trailing_newline: bool,
+}
+
+#[derive(Clone, Debug)]
+struct ExcerptIdMapping {
+    id: ExcerptId,
+    locator: Locator,
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct ExcerptRange<T> {
+    pub context: Range<T>,
+    pub primary: Option<Range<T>>,
+}
+
+#[derive(Clone, Debug, Default)]
+struct ExcerptSummary {
+    excerpt_id: ExcerptId,
+    excerpt_locator: Locator,
+    max_buffer_row: u32,
+    text: TextSummary,
+}
+
+#[derive(Clone)]
+pub struct MultiBufferRows<'a> {
+    buffer_row_range: Range<u32>,
+    excerpts: Cursor<'a, Excerpt, Point>,
+}
+
+pub struct MultiBufferChunks<'a> {
+    range: Range<usize>,
+    excerpts: Cursor<'a, Excerpt, usize>,
+    excerpt_chunks: Option<ExcerptChunks<'a>>,
+    language_aware: bool,
+}
+
+pub struct MultiBufferBytes<'a> {
+    range: Range<usize>,
+    excerpts: Cursor<'a, Excerpt, usize>,
+    excerpt_bytes: Option<ExcerptBytes<'a>>,
+    chunk: &'a [u8],
+}
+
+pub struct ReversedMultiBufferBytes<'a> {
+    range: Range<usize>,
+    excerpts: Cursor<'a, Excerpt, usize>,
+    excerpt_bytes: Option<ExcerptBytes<'a>>,
+    chunk: &'a [u8],
+}
+
+struct ExcerptChunks<'a> {
+    content_chunks: BufferChunks<'a>,
+    footer_height: usize,
+}
+
+struct ExcerptBytes<'a> {
+    content_bytes: text::Bytes<'a>,
+    footer_height: usize,
+}
+
+impl MultiBuffer {
+    pub fn new(replica_id: ReplicaId) -> Self {
+        Self {
+            snapshot: Default::default(),
+            buffers: Default::default(),
+            next_excerpt_id: 1,
+            subscriptions: Default::default(),
+            singleton: false,
+            replica_id,
+            history: History {
+                next_transaction_id: Default::default(),
+                undo_stack: Default::default(),
+                redo_stack: Default::default(),
+                transaction_depth: 0,
+                group_interval: Duration::from_millis(300),
+            },
+            title: Default::default(),
+        }
+    }
+
+    pub fn clone(&self, new_cx: &mut ModelContext<Self>) -> Self {
+        let mut buffers = HashMap::default();
+        for (buffer_id, buffer_state) in self.buffers.borrow().iter() {
+            buffers.insert(
+                *buffer_id,
+                BufferState {
+                    buffer: buffer_state.buffer.clone(),
+                    last_version: buffer_state.last_version.clone(),
+                    last_parse_count: buffer_state.last_parse_count,
+                    last_selections_update_count: buffer_state.last_selections_update_count,
+                    last_diagnostics_update_count: buffer_state.last_diagnostics_update_count,
+                    last_file_update_count: buffer_state.last_file_update_count,
+                    last_git_diff_update_count: buffer_state.last_git_diff_update_count,
+                    excerpts: buffer_state.excerpts.clone(),
+                    _subscriptions: [
+                        new_cx.observe(&buffer_state.buffer, |_, _, cx| cx.notify()),
+                        new_cx.subscribe(&buffer_state.buffer, Self::on_buffer_event),
+                    ],
+                },
+            );
+        }
+        Self {
+            snapshot: RefCell::new(self.snapshot.borrow().clone()),
+            buffers: RefCell::new(buffers),
+            next_excerpt_id: 1,
+            subscriptions: Default::default(),
+            singleton: self.singleton,
+            replica_id: self.replica_id,
+            history: self.history.clone(),
+            title: self.title.clone(),
+        }
+    }
+
+    pub fn with_title(mut self, title: String) -> Self {
+        self.title = Some(title);
+        self
+    }
+
+    pub fn singleton(buffer: Model<Buffer>, cx: &mut ModelContext<Self>) -> Self {
+        let mut this = Self::new(buffer.read(cx).replica_id());
+        this.singleton = true;
+        this.push_excerpts(
+            buffer,
+            [ExcerptRange {
+                context: text::Anchor::MIN..text::Anchor::MAX,
+                primary: None,
+            }],
+            cx,
+        );
+        this.snapshot.borrow_mut().singleton = true;
+        this
+    }
+
+    pub fn replica_id(&self) -> ReplicaId {
+        self.replica_id
+    }
+
+    pub fn snapshot(&self, cx: &AppContext) -> MultiBufferSnapshot {
+        self.sync(cx);
+        self.snapshot.borrow().clone()
+    }
+
+    pub fn read(&self, cx: &AppContext) -> Ref<MultiBufferSnapshot> {
+        self.sync(cx);
+        self.snapshot.borrow()
+    }
+
+    pub fn as_singleton(&self) -> Option<Model<Buffer>> {
+        if self.singleton {
+            return Some(
+                self.buffers
+                    .borrow()
+                    .values()
+                    .next()
+                    .unwrap()
+                    .buffer
+                    .clone(),
+            );
+        } else {
+            None
+        }
+    }
+
+    pub fn is_singleton(&self) -> bool {
+        self.singleton
+    }
+
+    pub fn subscribe(&mut self) -> Subscription {
+        self.subscriptions.subscribe()
+    }
+
+    pub fn is_dirty(&self, cx: &AppContext) -> bool {
+        self.read(cx).is_dirty()
+    }
+
+    pub fn has_conflict(&self, cx: &AppContext) -> bool {
+        self.read(cx).has_conflict()
+    }
+
+    // The `is_empty` signature doesn't match what clippy expects
+    #[allow(clippy::len_without_is_empty)]
+    pub fn len(&self, cx: &AppContext) -> usize {
+        self.read(cx).len()
+    }
+
+    pub fn is_empty(&self, cx: &AppContext) -> bool {
+        self.len(cx) != 0
+    }
+
+    pub fn symbols_containing<T: ToOffset>(
+        &self,
+        offset: T,
+        theme: Option<&SyntaxTheme>,
+        cx: &AppContext,
+    ) -> Option<(u64, Vec<OutlineItem<Anchor>>)> {
+        self.read(cx).symbols_containing(offset, theme)
+    }
+
+    pub fn edit<I, S, T>(
+        &mut self,
+        edits: I,
+        mut autoindent_mode: Option<AutoindentMode>,
+        cx: &mut ModelContext<Self>,
+    ) where
+        I: IntoIterator<Item = (Range<S>, T)>,
+        S: ToOffset,
+        T: Into<Arc<str>>,
+    {
+        if self.buffers.borrow().is_empty() {
+            return;
+        }
+
+        let snapshot = self.read(cx);
+        let edits = edits.into_iter().map(|(range, new_text)| {
+            let mut range = range.start.to_offset(&snapshot)..range.end.to_offset(&snapshot);
+            if range.start > range.end {
+                mem::swap(&mut range.start, &mut range.end);
+            }
+            (range, new_text)
+        });
+
+        if let Some(buffer) = self.as_singleton() {
+            return buffer.update(cx, |buffer, cx| {
+                buffer.edit(edits, autoindent_mode, cx);
+            });
+        }
+
+        let original_indent_columns = match &mut autoindent_mode {
+            Some(AutoindentMode::Block {
+                original_indent_columns,
+            }) => mem::take(original_indent_columns),
+            _ => Default::default(),
+        };
+
+        struct BufferEdit {
+            range: Range<usize>,
+            new_text: Arc<str>,
+            is_insertion: bool,
+            original_indent_column: u32,
+        }
+        let mut buffer_edits: HashMap<u64, Vec<BufferEdit>> = Default::default();
+        let mut edited_excerpt_ids = Vec::new();
+        let mut cursor = snapshot.excerpts.cursor::<usize>();
+        for (ix, (range, new_text)) in edits.enumerate() {
+            let new_text: Arc<str> = new_text.into();
+            let original_indent_column = original_indent_columns.get(ix).copied().unwrap_or(0);
+            cursor.seek(&range.start, Bias::Right, &());
+            if cursor.item().is_none() && range.start == *cursor.start() {
+                cursor.prev(&());
+            }
+            let start_excerpt = cursor.item().expect("start offset out of bounds");
+            let start_overshoot = range.start - cursor.start();
+            let buffer_start = start_excerpt
+                .range
+                .context
+                .start
+                .to_offset(&start_excerpt.buffer)
+                + start_overshoot;
+            edited_excerpt_ids.push(start_excerpt.id);
+
+            cursor.seek(&range.end, Bias::Right, &());
+            if cursor.item().is_none() && range.end == *cursor.start() {
+                cursor.prev(&());
+            }
+            let end_excerpt = cursor.item().expect("end offset out of bounds");
+            let end_overshoot = range.end - cursor.start();
+            let buffer_end = end_excerpt
+                .range
+                .context
+                .start
+                .to_offset(&end_excerpt.buffer)
+                + end_overshoot;
+
+            if start_excerpt.id == end_excerpt.id {
+                buffer_edits
+                    .entry(start_excerpt.buffer_id)
+                    .or_insert(Vec::new())
+                    .push(BufferEdit {
+                        range: buffer_start..buffer_end,
+                        new_text,
+                        is_insertion: true,
+                        original_indent_column,
+                    });
+            } else {
+                edited_excerpt_ids.push(end_excerpt.id);
+                let start_excerpt_range = buffer_start
+                    ..start_excerpt
+                        .range
+                        .context
+                        .end
+                        .to_offset(&start_excerpt.buffer);
+                let end_excerpt_range = end_excerpt
+                    .range
+                    .context
+                    .start
+                    .to_offset(&end_excerpt.buffer)
+                    ..buffer_end;
+                buffer_edits
+                    .entry(start_excerpt.buffer_id)
+                    .or_insert(Vec::new())
+                    .push(BufferEdit {
+                        range: start_excerpt_range,
+                        new_text: new_text.clone(),
+                        is_insertion: true,
+                        original_indent_column,
+                    });
+                buffer_edits
+                    .entry(end_excerpt.buffer_id)
+                    .or_insert(Vec::new())
+                    .push(BufferEdit {
+                        range: end_excerpt_range,
+                        new_text: new_text.clone(),
+                        is_insertion: false,
+                        original_indent_column,
+                    });
+
+                cursor.seek(&range.start, Bias::Right, &());
+                cursor.next(&());
+                while let Some(excerpt) = cursor.item() {
+                    if excerpt.id == end_excerpt.id {
+                        break;
+                    }
+                    buffer_edits
+                        .entry(excerpt.buffer_id)
+                        .or_insert(Vec::new())
+                        .push(BufferEdit {
+                            range: excerpt.range.context.to_offset(&excerpt.buffer),
+                            new_text: new_text.clone(),
+                            is_insertion: false,
+                            original_indent_column,
+                        });
+                    edited_excerpt_ids.push(excerpt.id);
+                    cursor.next(&());
+                }
+            }
+        }
+
+        drop(cursor);
+        drop(snapshot);
+        // Non-generic part of edit, hoisted out to avoid blowing up LLVM IR.
+        fn tail(
+            this: &mut MultiBuffer,
+            buffer_edits: HashMap<u64, Vec<BufferEdit>>,
+            autoindent_mode: Option<AutoindentMode>,
+            edited_excerpt_ids: Vec<ExcerptId>,
+            cx: &mut ModelContext<MultiBuffer>,
+        ) {
+            for (buffer_id, mut edits) in buffer_edits {
+                edits.sort_unstable_by_key(|edit| edit.range.start);
+                this.buffers.borrow()[&buffer_id]
+                    .buffer
+                    .update(cx, |buffer, cx| {
+                        let mut edits = edits.into_iter().peekable();
+                        let mut insertions = Vec::new();
+                        let mut original_indent_columns = Vec::new();
+                        let mut deletions = Vec::new();
+                        let empty_str: Arc<str> = "".into();
+                        while let Some(BufferEdit {
+                            mut range,
+                            new_text,
+                            mut is_insertion,
+                            original_indent_column,
+                        }) = edits.next()
+                        {
+                            while let Some(BufferEdit {
+                                range: next_range,
+                                is_insertion: next_is_insertion,
+                                ..
+                            }) = edits.peek()
+                            {
+                                if range.end >= next_range.start {
+                                    range.end = cmp::max(next_range.end, range.end);
+                                    is_insertion |= *next_is_insertion;
+                                    edits.next();
+                                } else {
+                                    break;
+                                }
+                            }
+
+                            if is_insertion {
+                                original_indent_columns.push(original_indent_column);
+                                insertions.push((
+                                    buffer.anchor_before(range.start)
+                                        ..buffer.anchor_before(range.end),
+                                    new_text.clone(),
+                                ));
+                            } else if !range.is_empty() {
+                                deletions.push((
+                                    buffer.anchor_before(range.start)
+                                        ..buffer.anchor_before(range.end),
+                                    empty_str.clone(),
+                                ));
+                            }
+                        }
+
+                        let deletion_autoindent_mode =
+                            if let Some(AutoindentMode::Block { .. }) = autoindent_mode {
+                                Some(AutoindentMode::Block {
+                                    original_indent_columns: Default::default(),
+                                })
+                            } else {
+                                None
+                            };
+                        let insertion_autoindent_mode =
+                            if let Some(AutoindentMode::Block { .. }) = autoindent_mode {
+                                Some(AutoindentMode::Block {
+                                    original_indent_columns,
+                                })
+                            } else {
+                                None
+                            };
+
+                        buffer.edit(deletions, deletion_autoindent_mode, cx);
+                        buffer.edit(insertions, insertion_autoindent_mode, cx);
+                    })
+            }
+
+            cx.emit(Event::ExcerptsEdited {
+                ids: edited_excerpt_ids,
+            });
+        }
+        tail(self, buffer_edits, autoindent_mode, edited_excerpt_ids, cx);
+    }
+
+    pub fn start_transaction(&mut self, cx: &mut ModelContext<Self>) -> Option<TransactionId> {
+        self.start_transaction_at(Instant::now(), cx)
+    }
+
+    pub fn start_transaction_at(
+        &mut self,
+        now: Instant,
+        cx: &mut ModelContext<Self>,
+    ) -> Option<TransactionId> {
+        if let Some(buffer) = self.as_singleton() {
+            return buffer.update(cx, |buffer, _| buffer.start_transaction_at(now));
+        }
+
+        for BufferState { buffer, .. } in self.buffers.borrow().values() {
+            buffer.update(cx, |buffer, _| buffer.start_transaction_at(now));
+        }
+        self.history.start_transaction(now)
+    }
+
+    pub fn end_transaction(&mut self, cx: &mut ModelContext<Self>) -> Option<TransactionId> {
+        self.end_transaction_at(Instant::now(), cx)
+    }
+
+    pub fn end_transaction_at(
+        &mut self,
+        now: Instant,
+        cx: &mut ModelContext<Self>,
+    ) -> Option<TransactionId> {
+        if let Some(buffer) = self.as_singleton() {
+            return buffer.update(cx, |buffer, cx| buffer.end_transaction_at(now, cx));
+        }
+
+        let mut buffer_transactions = HashMap::default();
+        for BufferState { buffer, .. } in self.buffers.borrow().values() {
+            if let Some(transaction_id) =
+                buffer.update(cx, |buffer, cx| buffer.end_transaction_at(now, cx))
+            {
+                buffer_transactions.insert(buffer.read(cx).remote_id(), transaction_id);
+            }
+        }
+
+        if self.history.end_transaction(now, buffer_transactions) {
+            let transaction_id = self.history.group().unwrap();
+            Some(transaction_id)
+        } else {
+            None
+        }
+    }
+
+    pub fn merge_transactions(
+        &mut self,
+        transaction: TransactionId,
+        destination: TransactionId,
+        cx: &mut ModelContext<Self>,
+    ) {
+        if let Some(buffer) = self.as_singleton() {
+            buffer.update(cx, |buffer, _| {
+                buffer.merge_transactions(transaction, destination)
+            });
+        } else {
+            if let Some(transaction) = self.history.forget(transaction) {
+                if let Some(destination) = self.history.transaction_mut(destination) {
+                    for (buffer_id, buffer_transaction_id) in transaction.buffer_transactions {
+                        if let Some(destination_buffer_transaction_id) =
+                            destination.buffer_transactions.get(&buffer_id)
+                        {
+                            if let Some(state) = self.buffers.borrow().get(&buffer_id) {
+                                state.buffer.update(cx, |buffer, _| {
+                                    buffer.merge_transactions(
+                                        buffer_transaction_id,
+                                        *destination_buffer_transaction_id,
+                                    )
+                                });
+                            }
+                        } else {
+                            destination
+                                .buffer_transactions
+                                .insert(buffer_id, buffer_transaction_id);
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    pub fn finalize_last_transaction(&mut self, cx: &mut ModelContext<Self>) {
+        self.history.finalize_last_transaction();
+        for BufferState { buffer, .. } in self.buffers.borrow().values() {
+            buffer.update(cx, |buffer, _| {
+                buffer.finalize_last_transaction();
+            });
+        }
+    }
+
+    pub fn push_transaction<'a, T>(&mut self, buffer_transactions: T, cx: &mut ModelContext<Self>)
+    where
+        T: IntoIterator<Item = (&'a Model<Buffer>, &'a language2::Transaction)>,
+    {
+        self.history
+            .push_transaction(buffer_transactions, Instant::now(), cx);
+        self.history.finalize_last_transaction();
+    }
+
+    pub fn group_until_transaction(
+        &mut self,
+        transaction_id: TransactionId,
+        cx: &mut ModelContext<Self>,
+    ) {
+        if let Some(buffer) = self.as_singleton() {
+            buffer.update(cx, |buffer, _| {
+                buffer.group_until_transaction(transaction_id)
+            });
+        } else {
+            self.history.group_until(transaction_id);
+        }
+    }
+
+    pub fn set_active_selections(
+        &mut self,
+        selections: &[Selection<Anchor>],
+        line_mode: bool,
+        cursor_shape: CursorShape,
+        cx: &mut ModelContext<Self>,
+    ) {
+        let mut selections_by_buffer: HashMap<u64, Vec<Selection<text::Anchor>>> =
+            Default::default();
+        let snapshot = self.read(cx);
+        let mut cursor = snapshot.excerpts.cursor::<Option<&Locator>>();
+        for selection in selections {
+            let start_locator = snapshot.excerpt_locator_for_id(selection.start.excerpt_id);
+            let end_locator = snapshot.excerpt_locator_for_id(selection.end.excerpt_id);
+
+            cursor.seek(&Some(start_locator), Bias::Left, &());
+            while let Some(excerpt) = cursor.item() {
+                if excerpt.locator > *end_locator {
+                    break;
+                }
+
+                let mut start = excerpt.range.context.start;
+                let mut end = excerpt.range.context.end;
+                if excerpt.id == selection.start.excerpt_id {
+                    start = selection.start.text_anchor;
+                }
+                if excerpt.id == selection.end.excerpt_id {
+                    end = selection.end.text_anchor;
+                }
+                selections_by_buffer
+                    .entry(excerpt.buffer_id)
+                    .or_default()
+                    .push(Selection {
+                        id: selection.id,
+                        start,
+                        end,
+                        reversed: selection.reversed,
+                        goal: selection.goal,
+                    });
+
+                cursor.next(&());
+            }
+        }
+
+        for (buffer_id, buffer_state) in self.buffers.borrow().iter() {
+            if !selections_by_buffer.contains_key(buffer_id) {
+                buffer_state
+                    .buffer
+                    .update(cx, |buffer, cx| buffer.remove_active_selections(cx));
+            }
+        }
+
+        for (buffer_id, mut selections) in selections_by_buffer {
+            self.buffers.borrow()[&buffer_id]
+                .buffer
+                .update(cx, |buffer, cx| {
+                    selections.sort_unstable_by(|a, b| a.start.cmp(&b.start, buffer));
+                    let mut selections = selections.into_iter().peekable();
+                    let merged_selections = Arc::from_iter(iter::from_fn(|| {
+                        let mut selection = selections.next()?;
+                        while let Some(next_selection) = selections.peek() {
+                            if selection.end.cmp(&next_selection.start, buffer).is_ge() {
+                                let next_selection = selections.next().unwrap();
+                                if next_selection.end.cmp(&selection.end, buffer).is_ge() {
+                                    selection.end = next_selection.end;
+                                }
+                            } else {
+                                break;
+                            }
+                        }
+                        Some(selection)
+                    }));
+                    buffer.set_active_selections(merged_selections, line_mode, cursor_shape, cx);
+                });
+        }
+    }
+
+    pub fn remove_active_selections(&mut self, cx: &mut ModelContext<Self>) {
+        for buffer in self.buffers.borrow().values() {
+            buffer
+                .buffer
+                .update(cx, |buffer, cx| buffer.remove_active_selections(cx));
+        }
+    }
+
+    pub fn undo(&mut self, cx: &mut ModelContext<Self>) -> Option<TransactionId> {
+        let mut transaction_id = None;
+        if let Some(buffer) = self.as_singleton() {
+            transaction_id = buffer.update(cx, |buffer, cx| buffer.undo(cx));
+        } else {
+            while let Some(transaction) = self.history.pop_undo() {
+                let mut undone = false;
+                for (buffer_id, buffer_transaction_id) in &mut transaction.buffer_transactions {
+                    if let Some(BufferState { buffer, .. }) = self.buffers.borrow().get(buffer_id) {
+                        undone |= buffer.update(cx, |buffer, cx| {
+                            let undo_to = *buffer_transaction_id;
+                            if let Some(entry) = buffer.peek_undo_stack() {
+                                *buffer_transaction_id = entry.transaction_id();
+                            }
+                            buffer.undo_to_transaction(undo_to, cx)
+                        });
+                    }
+                }
+
+                if undone {
+                    transaction_id = Some(transaction.id);
+                    break;
+                }
+            }
+        }
+
+        if let Some(transaction_id) = transaction_id {
+            cx.emit(Event::TransactionUndone { transaction_id });
+        }
+
+        transaction_id
+    }
+
+    pub fn redo(&mut self, cx: &mut ModelContext<Self>) -> Option<TransactionId> {
+        if let Some(buffer) = self.as_singleton() {
+            return buffer.update(cx, |buffer, cx| buffer.redo(cx));
+        }
+
+        while let Some(transaction) = self.history.pop_redo() {
+            let mut redone = false;
+            for (buffer_id, buffer_transaction_id) in &mut transaction.buffer_transactions {
+                if let Some(BufferState { buffer, .. }) = self.buffers.borrow().get(buffer_id) {
+                    redone |= buffer.update(cx, |buffer, cx| {
+                        let redo_to = *buffer_transaction_id;
+                        if let Some(entry) = buffer.peek_redo_stack() {
+                            *buffer_transaction_id = entry.transaction_id();
+                        }
+                        buffer.redo_to_transaction(redo_to, cx)
+                    });
+                }
+            }
+
+            if redone {
+                return Some(transaction.id);
+            }
+        }
+
+        None
+    }
+
+    pub fn undo_transaction(&mut self, transaction_id: TransactionId, cx: &mut ModelContext<Self>) {
+        if let Some(buffer) = self.as_singleton() {
+            buffer.update(cx, |buffer, cx| buffer.undo_transaction(transaction_id, cx));
+        } else if let Some(transaction) = self.history.remove_from_undo(transaction_id) {
+            for (buffer_id, transaction_id) in &transaction.buffer_transactions {
+                if let Some(BufferState { buffer, .. }) = self.buffers.borrow().get(buffer_id) {
+                    buffer.update(cx, |buffer, cx| {
+                        buffer.undo_transaction(*transaction_id, cx)
+                    });
+                }
+            }
+        }
+    }
+
+    pub fn stream_excerpts_with_context_lines(
+        &mut self,
+        buffer: Model<Buffer>,
+        ranges: Vec<Range<text::Anchor>>,
+        context_line_count: u32,
+        cx: &mut ModelContext<Self>,
+    ) -> mpsc::Receiver<Range<Anchor>> {
+        let (buffer_id, buffer_snapshot) =
+            buffer.update(cx, |buffer, _| (buffer.remote_id(), buffer.snapshot()));
+
+        let (mut tx, rx) = mpsc::channel(256);
+        cx.spawn(move |this, mut cx| async move {
+            let mut excerpt_ranges = Vec::new();
+            let mut range_counts = Vec::new();
+            cx.executor()
+                .scoped(|scope| {
+                    scope.spawn(async {
+                        let (ranges, counts) =
+                            build_excerpt_ranges(&buffer_snapshot, &ranges, context_line_count);
+                        excerpt_ranges = ranges;
+                        range_counts = counts;
+                    });
+                })
+                .await;
+
+            let mut ranges = ranges.into_iter();
+            let mut range_counts = range_counts.into_iter();
+            for excerpt_ranges in excerpt_ranges.chunks(100) {
+                let excerpt_ids = match this.update(&mut cx, |this, cx| {
+                    this.push_excerpts(buffer.clone(), excerpt_ranges.iter().cloned(), cx)
+                }) {
+                    Ok(excerpt_ids) => excerpt_ids,
+                    Err(_) => return,
+                };
+
+                for (excerpt_id, range_count) in excerpt_ids.into_iter().zip(range_counts.by_ref())
+                {
+                    for range in ranges.by_ref().take(range_count) {
+                        let start = Anchor {
+                            buffer_id: Some(buffer_id),
+                            excerpt_id: excerpt_id.clone(),
+                            text_anchor: range.start,
+                        };
+                        let end = Anchor {
+                            buffer_id: Some(buffer_id),
+                            excerpt_id: excerpt_id.clone(),
+                            text_anchor: range.end,
+                        };
+                        if tx.send(start..end).await.is_err() {
+                            break;
+                        }
+                    }
+                }
+            }
+        })
+        .detach();
+
+        rx
+    }
+
+    pub fn push_excerpts<O>(
+        &mut self,
+        buffer: Model<Buffer>,
+        ranges: impl IntoIterator<Item = ExcerptRange<O>>,
+        cx: &mut ModelContext<Self>,
+    ) -> Vec<ExcerptId>
+    where
+        O: text::ToOffset,
+    {
+        self.insert_excerpts_after(ExcerptId::max(), buffer, ranges, cx)
+    }
+
+    pub fn push_excerpts_with_context_lines<O>(
+        &mut self,
+        buffer: Model<Buffer>,
+        ranges: Vec<Range<O>>,
+        context_line_count: u32,
+        cx: &mut ModelContext<Self>,
+    ) -> Vec<Range<Anchor>>
+    where
+        O: text::ToPoint + text::ToOffset,
+    {
+        let buffer_id = buffer.read(cx).remote_id();
+        let buffer_snapshot = buffer.read(cx).snapshot();
+        let (excerpt_ranges, range_counts) =
+            build_excerpt_ranges(&buffer_snapshot, &ranges, context_line_count);
+
+        let excerpt_ids = self.push_excerpts(buffer, excerpt_ranges, cx);
+
+        let mut anchor_ranges = Vec::new();
+        let mut ranges = ranges.into_iter();
+        for (excerpt_id, range_count) in excerpt_ids.into_iter().zip(range_counts.into_iter()) {
+            anchor_ranges.extend(ranges.by_ref().take(range_count).map(|range| {
+                let start = Anchor {
+                    buffer_id: Some(buffer_id),
+                    excerpt_id: excerpt_id.clone(),
+                    text_anchor: buffer_snapshot.anchor_after(range.start),
+                };
+                let end = Anchor {
+                    buffer_id: Some(buffer_id),
+                    excerpt_id: excerpt_id.clone(),
+                    text_anchor: buffer_snapshot.anchor_after(range.end),
+                };
+                start..end
+            }))
+        }
+        anchor_ranges
+    }
+
+    pub fn insert_excerpts_after<O>(
+        &mut self,
+        prev_excerpt_id: ExcerptId,
+        buffer: Model<Buffer>,
+        ranges: impl IntoIterator<Item = ExcerptRange<O>>,
+        cx: &mut ModelContext<Self>,
+    ) -> Vec<ExcerptId>
+    where
+        O: text::ToOffset,
+    {
+        let mut ids = Vec::new();
+        let mut next_excerpt_id = self.next_excerpt_id;
+        self.insert_excerpts_with_ids_after(
+            prev_excerpt_id,
+            buffer,
+            ranges.into_iter().map(|range| {
+                let id = ExcerptId(post_inc(&mut next_excerpt_id));
+                ids.push(id);
+                (id, range)
+            }),
+            cx,
+        );
+        ids
+    }
+
+    pub fn insert_excerpts_with_ids_after<O>(
+        &mut self,
+        prev_excerpt_id: ExcerptId,
+        buffer: Model<Buffer>,
+        ranges: impl IntoIterator<Item = (ExcerptId, ExcerptRange<O>)>,
+        cx: &mut ModelContext<Self>,
+    ) where
+        O: text::ToOffset,
+    {
+        assert_eq!(self.history.transaction_depth, 0);
+        let mut ranges = ranges.into_iter().peekable();
+        if ranges.peek().is_none() {
+            return Default::default();
+        }
+
+        self.sync(cx);
+
+        let buffer_id = buffer.read(cx).remote_id();
+        let buffer_snapshot = buffer.read(cx).snapshot();
+
+        let mut buffers = self.buffers.borrow_mut();
+        let buffer_state = buffers.entry(buffer_id).or_insert_with(|| BufferState {
+            last_version: buffer_snapshot.version().clone(),
+            last_parse_count: buffer_snapshot.parse_count(),
+            last_selections_update_count: buffer_snapshot.selections_update_count(),
+            last_diagnostics_update_count: buffer_snapshot.diagnostics_update_count(),
+            last_file_update_count: buffer_snapshot.file_update_count(),
+            last_git_diff_update_count: buffer_snapshot.git_diff_update_count(),
+            excerpts: Default::default(),
+            _subscriptions: [
+                cx.observe(&buffer, |_, _, cx| cx.notify()),
+                cx.subscribe(&buffer, Self::on_buffer_event),
+            ],
+            buffer: buffer.clone(),
+        });
+
+        let mut snapshot = self.snapshot.borrow_mut();
+
+        let mut prev_locator = snapshot.excerpt_locator_for_id(prev_excerpt_id).clone();
+        let mut new_excerpt_ids = mem::take(&mut snapshot.excerpt_ids);
+        let mut cursor = snapshot.excerpts.cursor::<Option<&Locator>>();
+        let mut new_excerpts = cursor.slice(&prev_locator, Bias::Right, &());
+        prev_locator = cursor.start().unwrap_or(Locator::min_ref()).clone();
+
+        let edit_start = new_excerpts.summary().text.len;
+        new_excerpts.update_last(
+            |excerpt| {
+                excerpt.has_trailing_newline = true;
+            },
+            &(),
+        );
+
+        let next_locator = if let Some(excerpt) = cursor.item() {
+            excerpt.locator.clone()
+        } else {
+            Locator::max()
+        };
+
+        let mut excerpts = Vec::new();
+        while let Some((id, range)) = ranges.next() {
+            let locator = Locator::between(&prev_locator, &next_locator);
+            if let Err(ix) = buffer_state.excerpts.binary_search(&locator) {
+                buffer_state.excerpts.insert(ix, locator.clone());
+            }
+            let range = ExcerptRange {
+                context: buffer_snapshot.anchor_before(&range.context.start)
+                    ..buffer_snapshot.anchor_after(&range.context.end),
+                primary: range.primary.map(|primary| {
+                    buffer_snapshot.anchor_before(&primary.start)
+                        ..buffer_snapshot.anchor_after(&primary.end)
+                }),
+            };
+            if id.0 >= self.next_excerpt_id {
+                self.next_excerpt_id = id.0 + 1;
+            }
+            excerpts.push((id, range.clone()));
+            let excerpt = Excerpt::new(
+                id,
+                locator.clone(),
+                buffer_id,
+                buffer_snapshot.clone(),
+                range,
+                ranges.peek().is_some() || cursor.item().is_some(),
+            );
+            new_excerpts.push(excerpt, &());
+            prev_locator = locator.clone();
+            new_excerpt_ids.push(ExcerptIdMapping { id, locator }, &());
+        }
+
+        let edit_end = new_excerpts.summary().text.len;
+
+        let suffix = cursor.suffix(&());
+        let changed_trailing_excerpt = suffix.is_empty();
+        new_excerpts.append(suffix, &());
+        drop(cursor);
+        snapshot.excerpts = new_excerpts;
+        snapshot.excerpt_ids = new_excerpt_ids;
+        if changed_trailing_excerpt {
+            snapshot.trailing_excerpt_update_count += 1;
+        }
+
+        self.subscriptions.publish_mut([Edit {
+            old: edit_start..edit_start,
+            new: edit_start..edit_end,
+        }]);
+        cx.emit(Event::Edited {
+            sigleton_buffer_edited: false,
+        });
+        cx.emit(Event::ExcerptsAdded {
+            buffer,
+            predecessor: prev_excerpt_id,
+            excerpts,
+        });
+        cx.notify();
+    }
+
+    pub fn clear(&mut self, cx: &mut ModelContext<Self>) {
+        self.sync(cx);
+        let ids = self.excerpt_ids();
+        self.buffers.borrow_mut().clear();
+        let mut snapshot = self.snapshot.borrow_mut();
+        let prev_len = snapshot.len();
+        snapshot.excerpts = Default::default();
+        snapshot.trailing_excerpt_update_count += 1;
+        snapshot.is_dirty = false;
+        snapshot.has_conflict = false;
+
+        self.subscriptions.publish_mut([Edit {
+            old: 0..prev_len,
+            new: 0..0,
+        }]);
+        cx.emit(Event::Edited {
+            sigleton_buffer_edited: false,
+        });
+        cx.emit(Event::ExcerptsRemoved { ids });
+        cx.notify();
+    }
+
+    pub fn excerpts_for_buffer(
+        &self,
+        buffer: &Model<Buffer>,
+        cx: &AppContext,
+    ) -> Vec<(ExcerptId, ExcerptRange<text::Anchor>)> {
+        let mut excerpts = Vec::new();
+        let snapshot = self.read(cx);
+        let buffers = self.buffers.borrow();
+        let mut cursor = snapshot.excerpts.cursor::<Option<&Locator>>();
+        for locator in buffers
+            .get(&buffer.read(cx).remote_id())
+            .map(|state| &state.excerpts)
+            .into_iter()
+            .flatten()
+        {
+            cursor.seek_forward(&Some(locator), Bias::Left, &());
+            if let Some(excerpt) = cursor.item() {
+                if excerpt.locator == *locator {
+                    excerpts.push((excerpt.id.clone(), excerpt.range.clone()));
+                }
+            }
+        }
+
+        excerpts
+    }
+
+    pub fn excerpt_ids(&self) -> Vec<ExcerptId> {
+        self.snapshot
+            .borrow()
+            .excerpts
+            .iter()
+            .map(|entry| entry.id)
+            .collect()
+    }
+
+    pub fn excerpt_containing(
+        &self,
+        position: impl ToOffset,
+        cx: &AppContext,
+    ) -> Option<(ExcerptId, Model<Buffer>, Range<text::Anchor>)> {
+        let snapshot = self.read(cx);
+        let position = position.to_offset(&snapshot);
+
+        let mut cursor = snapshot.excerpts.cursor::<usize>();
+        cursor.seek(&position, Bias::Right, &());
+        cursor
+            .item()
+            .or_else(|| snapshot.excerpts.last())
+            .map(|excerpt| {
+                (
+                    excerpt.id.clone(),
+                    self.buffers
+                        .borrow()
+                        .get(&excerpt.buffer_id)
+                        .unwrap()
+                        .buffer
+                        .clone(),
+                    excerpt.range.context.clone(),
+                )
+            })
+    }
+
+    // If point is at the end of the buffer, the last excerpt is returned
+    pub fn point_to_buffer_offset<T: ToOffset>(
+        &self,
+        point: T,
+        cx: &AppContext,
+    ) -> Option<(Model<Buffer>, usize, ExcerptId)> {
+        let snapshot = self.read(cx);
+        let offset = point.to_offset(&snapshot);
+        let mut cursor = snapshot.excerpts.cursor::<usize>();
+        cursor.seek(&offset, Bias::Right, &());
+        if cursor.item().is_none() {
+            cursor.prev(&());
+        }
+
+        cursor.item().map(|excerpt| {
+            let excerpt_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let buffer_point = excerpt_start + offset - *cursor.start();
+            let buffer = self.buffers.borrow()[&excerpt.buffer_id].buffer.clone();
+
+            (buffer, buffer_point, excerpt.id)
+        })
+    }
+
+    pub fn range_to_buffer_ranges<T: ToOffset>(
+        &self,
+        range: Range<T>,
+        cx: &AppContext,
+    ) -> Vec<(Model<Buffer>, Range<usize>, ExcerptId)> {
+        let snapshot = self.read(cx);
+        let start = range.start.to_offset(&snapshot);
+        let end = range.end.to_offset(&snapshot);
+
+        let mut result = Vec::new();
+        let mut cursor = snapshot.excerpts.cursor::<usize>();
+        cursor.seek(&start, Bias::Right, &());
+        if cursor.item().is_none() {
+            cursor.prev(&());
+        }
+
+        while let Some(excerpt) = cursor.item() {
+            if *cursor.start() > end {
+                break;
+            }
+
+            let mut end_before_newline = cursor.end(&());
+            if excerpt.has_trailing_newline {
+                end_before_newline -= 1;
+            }
+            let excerpt_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let start = excerpt_start + (cmp::max(start, *cursor.start()) - *cursor.start());
+            let end = excerpt_start + (cmp::min(end, end_before_newline) - *cursor.start());
+            let buffer = self.buffers.borrow()[&excerpt.buffer_id].buffer.clone();
+            result.push((buffer, start..end, excerpt.id));
+            cursor.next(&());
+        }
+
+        result
+    }
+
+    pub fn remove_excerpts(
+        &mut self,
+        excerpt_ids: impl IntoIterator<Item = ExcerptId>,
+        cx: &mut ModelContext<Self>,
+    ) {
+        self.sync(cx);
+        let ids = excerpt_ids.into_iter().collect::<Vec<_>>();
+        if ids.is_empty() {
+            return;
+        }
+
+        let mut buffers = self.buffers.borrow_mut();
+        let mut snapshot = self.snapshot.borrow_mut();
+        let mut new_excerpts = SumTree::new();
+        let mut cursor = snapshot.excerpts.cursor::<(Option<&Locator>, usize)>();
+        let mut edits = Vec::new();
+        let mut excerpt_ids = ids.iter().copied().peekable();
+
+        while let Some(excerpt_id) = excerpt_ids.next() {
+            // Seek to the next excerpt to remove, preserving any preceding excerpts.
+            let locator = snapshot.excerpt_locator_for_id(excerpt_id);
+            new_excerpts.append(cursor.slice(&Some(locator), Bias::Left, &()), &());
+
+            if let Some(mut excerpt) = cursor.item() {
+                if excerpt.id != excerpt_id {
+                    continue;
+                }
+                let mut old_start = cursor.start().1;
+
+                // Skip over the removed excerpt.
+                'remove_excerpts: loop {
+                    if let Some(buffer_state) = buffers.get_mut(&excerpt.buffer_id) {
+                        buffer_state.excerpts.retain(|l| l != &excerpt.locator);
+                        if buffer_state.excerpts.is_empty() {
+                            buffers.remove(&excerpt.buffer_id);
+                        }
+                    }
+                    cursor.next(&());
+
+                    // Skip over any subsequent excerpts that are also removed.
+                    while let Some(&next_excerpt_id) = excerpt_ids.peek() {
+                        let next_locator = snapshot.excerpt_locator_for_id(next_excerpt_id);
+                        if let Some(next_excerpt) = cursor.item() {
+                            if next_excerpt.locator == *next_locator {
+                                excerpt_ids.next();
+                                excerpt = next_excerpt;
+                                continue 'remove_excerpts;
+                            }
+                        }
+                        break;
+                    }
+
+                    break;
+                }
+
+                // When removing the last excerpt, remove the trailing newline from
+                // the previous excerpt.
+                if cursor.item().is_none() && old_start > 0 {
+                    old_start -= 1;
+                    new_excerpts.update_last(|e| e.has_trailing_newline = false, &());
+                }
+
+                // Push an edit for the removal of this run of excerpts.
+                let old_end = cursor.start().1;
+                let new_start = new_excerpts.summary().text.len;
+                edits.push(Edit {
+                    old: old_start..old_end,
+                    new: new_start..new_start,
+                });
+            }
+        }
+        let suffix = cursor.suffix(&());
+        let changed_trailing_excerpt = suffix.is_empty();
+        new_excerpts.append(suffix, &());
+        drop(cursor);
+        snapshot.excerpts = new_excerpts;
+
+        if changed_trailing_excerpt {
+            snapshot.trailing_excerpt_update_count += 1;
+        }
+
+        self.subscriptions.publish_mut(edits);
+        cx.emit(Event::Edited {
+            sigleton_buffer_edited: false,
+        });
+        cx.emit(Event::ExcerptsRemoved { ids });
+        cx.notify();
+    }
+
+    pub fn wait_for_anchors<'a>(
+        &self,
+        anchors: impl 'a + Iterator<Item = Anchor>,
+        cx: &mut ModelContext<Self>,
+    ) -> impl 'static + Future<Output = Result<()>> {
+        let borrow = self.buffers.borrow();
+        let mut error = None;
+        let mut futures = Vec::new();
+        for anchor in anchors {
+            if let Some(buffer_id) = anchor.buffer_id {
+                if let Some(buffer) = borrow.get(&buffer_id) {
+                    buffer.buffer.update(cx, |buffer, _| {
+                        futures.push(buffer.wait_for_anchors([anchor.text_anchor]))
+                    });
+                } else {
+                    error = Some(anyhow!(
+                        "buffer {buffer_id} is not part of this multi-buffer"
+                    ));
+                    break;
+                }
+            }
+        }
+        async move {
+            if let Some(error) = error {
+                Err(error)?;
+            }
+            for future in futures {
+                future.await?;
+            }
+            Ok(())
+        }
+    }
+
+    pub fn text_anchor_for_position<T: ToOffset>(
+        &self,
+        position: T,
+        cx: &AppContext,
+    ) -> Option<(Model<Buffer>, language2::Anchor)> {
+        let snapshot = self.read(cx);
+        let anchor = snapshot.anchor_before(position);
+        let buffer = self
+            .buffers
+            .borrow()
+            .get(&anchor.buffer_id?)?
+            .buffer
+            .clone();
+        Some((buffer, anchor.text_anchor))
+    }
+
+    fn on_buffer_event(
+        &mut self,
+        _: Model<Buffer>,
+        event: &language2::Event,
+        cx: &mut ModelContext<Self>,
+    ) {
+        cx.emit(match event {
+            language2::Event::Edited => Event::Edited {
+                sigleton_buffer_edited: true,
+            },
+            language2::Event::DirtyChanged => Event::DirtyChanged,
+            language2::Event::Saved => Event::Saved,
+            language2::Event::FileHandleChanged => Event::FileHandleChanged,
+            language2::Event::Reloaded => Event::Reloaded,
+            language2::Event::DiffBaseChanged => Event::DiffBaseChanged,
+            language2::Event::LanguageChanged => Event::LanguageChanged,
+            language2::Event::Reparsed => Event::Reparsed,
+            language2::Event::DiagnosticsUpdated => Event::DiagnosticsUpdated,
+            language2::Event::Closed => Event::Closed,
+
+            //
+            language2::Event::Operation(_) => return,
+        });
+    }
+
+    pub fn all_buffers(&self) -> HashSet<Model<Buffer>> {
+        self.buffers
+            .borrow()
+            .values()
+            .map(|state| state.buffer.clone())
+            .collect()
+    }
+
+    pub fn buffer(&self, buffer_id: u64) -> Option<Model<Buffer>> {
+        self.buffers
+            .borrow()
+            .get(&buffer_id)
+            .map(|state| state.buffer.clone())
+    }
+
+    pub fn is_completion_trigger(&self, position: Anchor, text: &str, cx: &AppContext) -> bool {
+        let mut chars = text.chars();
+        let char = if let Some(char) = chars.next() {
+            char
+        } else {
+            return false;
+        };
+        if chars.next().is_some() {
+            return false;
+        }
+
+        let snapshot = self.snapshot(cx);
+        let position = position.to_offset(&snapshot);
+        let scope = snapshot.language_scope_at(position);
+        if char_kind(&scope, char) == CharKind::Word {
+            return true;
+        }
+
+        let anchor = snapshot.anchor_before(position);
+        anchor
+            .buffer_id
+            .and_then(|buffer_id| {
+                let buffer = self.buffers.borrow().get(&buffer_id)?.buffer.clone();
+                Some(
+                    buffer
+                        .read(cx)
+                        .completion_triggers()
+                        .iter()
+                        .any(|string| string == text),
+                )
+            })
+            .unwrap_or(false)
+    }
+
+    pub fn language_at<'a, T: ToOffset>(
+        &self,
+        point: T,
+        cx: &'a AppContext,
+    ) -> Option<Arc<Language>> {
+        self.point_to_buffer_offset(point, cx)
+            .and_then(|(buffer, offset, _)| buffer.read(cx).language_at(offset))
+    }
+
+    pub fn settings_at<'a, T: ToOffset>(
+        &self,
+        point: T,
+        cx: &'a AppContext,
+    ) -> &'a LanguageSettings {
+        let mut language = None;
+        let mut file = None;
+        if let Some((buffer, offset, _)) = self.point_to_buffer_offset(point, cx) {
+            let buffer = buffer.read(cx);
+            language = buffer.language_at(offset);
+            file = buffer.file();
+        }
+        language_settings(language.as_ref(), file, cx)
+    }
+
+    pub fn for_each_buffer(&self, mut f: impl FnMut(&Model<Buffer>)) {
+        self.buffers
+            .borrow()
+            .values()
+            .for_each(|state| f(&state.buffer))
+    }
+
+    pub fn title<'a>(&'a self, cx: &'a AppContext) -> Cow<'a, str> {
+        if let Some(title) = self.title.as_ref() {
+            return title.into();
+        }
+
+        if let Some(buffer) = self.as_singleton() {
+            if let Some(file) = buffer.read(cx).file() {
+                return file.file_name(cx).to_string_lossy();
+            }
+        }
+
+        "untitled".into()
+    }
+
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn is_parsing(&self, cx: &AppContext) -> bool {
+        self.as_singleton().unwrap().read(cx).is_parsing()
+    }
+
+    fn sync(&self, cx: &AppContext) {
+        let mut snapshot = self.snapshot.borrow_mut();
+        let mut excerpts_to_edit = Vec::new();
+        let mut reparsed = false;
+        let mut diagnostics_updated = false;
+        let mut git_diff_updated = false;
+        let mut is_dirty = false;
+        let mut has_conflict = false;
+        let mut edited = false;
+        let mut buffers = self.buffers.borrow_mut();
+        for buffer_state in buffers.values_mut() {
+            let buffer = buffer_state.buffer.read(cx);
+            let version = buffer.version();
+            let parse_count = buffer.parse_count();
+            let selections_update_count = buffer.selections_update_count();
+            let diagnostics_update_count = buffer.diagnostics_update_count();
+            let file_update_count = buffer.file_update_count();
+            let git_diff_update_count = buffer.git_diff_update_count();
+
+            let buffer_edited = version.changed_since(&buffer_state.last_version);
+            let buffer_reparsed = parse_count > buffer_state.last_parse_count;
+            let buffer_selections_updated =
+                selections_update_count > buffer_state.last_selections_update_count;
+            let buffer_diagnostics_updated =
+                diagnostics_update_count > buffer_state.last_diagnostics_update_count;
+            let buffer_file_updated = file_update_count > buffer_state.last_file_update_count;
+            let buffer_git_diff_updated =
+                git_diff_update_count > buffer_state.last_git_diff_update_count;
+            if buffer_edited
+                || buffer_reparsed
+                || buffer_selections_updated
+                || buffer_diagnostics_updated
+                || buffer_file_updated
+                || buffer_git_diff_updated
+            {
+                buffer_state.last_version = version;
+                buffer_state.last_parse_count = parse_count;
+                buffer_state.last_selections_update_count = selections_update_count;
+                buffer_state.last_diagnostics_update_count = diagnostics_update_count;
+                buffer_state.last_file_update_count = file_update_count;
+                buffer_state.last_git_diff_update_count = git_diff_update_count;
+                excerpts_to_edit.extend(
+                    buffer_state
+                        .excerpts
+                        .iter()
+                        .map(|locator| (locator, buffer_state.buffer.clone(), buffer_edited)),
+                );
+            }
+
+            edited |= buffer_edited;
+            reparsed |= buffer_reparsed;
+            diagnostics_updated |= buffer_diagnostics_updated;
+            git_diff_updated |= buffer_git_diff_updated;
+            is_dirty |= buffer.is_dirty();
+            has_conflict |= buffer.has_conflict();
+        }
+        if edited {
+            snapshot.edit_count += 1;
+        }
+        if reparsed {
+            snapshot.parse_count += 1;
+        }
+        if diagnostics_updated {
+            snapshot.diagnostics_update_count += 1;
+        }
+        if git_diff_updated {
+            snapshot.git_diff_update_count += 1;
+        }
+        snapshot.is_dirty = is_dirty;
+        snapshot.has_conflict = has_conflict;
+
+        excerpts_to_edit.sort_unstable_by_key(|(locator, _, _)| *locator);
+
+        let mut edits = Vec::new();
+        let mut new_excerpts = SumTree::new();
+        let mut cursor = snapshot.excerpts.cursor::<(Option<&Locator>, usize)>();
+
+        for (locator, buffer, buffer_edited) in excerpts_to_edit {
+            new_excerpts.append(cursor.slice(&Some(locator), Bias::Left, &()), &());
+            let old_excerpt = cursor.item().unwrap();
+            let buffer = buffer.read(cx);
+            let buffer_id = buffer.remote_id();
+
+            let mut new_excerpt;
+            if buffer_edited {
+                edits.extend(
+                    buffer
+                        .edits_since_in_range::<usize>(
+                            old_excerpt.buffer.version(),
+                            old_excerpt.range.context.clone(),
+                        )
+                        .map(|mut edit| {
+                            let excerpt_old_start = cursor.start().1;
+                            let excerpt_new_start = new_excerpts.summary().text.len;
+                            edit.old.start += excerpt_old_start;
+                            edit.old.end += excerpt_old_start;
+                            edit.new.start += excerpt_new_start;
+                            edit.new.end += excerpt_new_start;
+                            edit
+                        }),
+                );
+
+                new_excerpt = Excerpt::new(
+                    old_excerpt.id,
+                    locator.clone(),
+                    buffer_id,
+                    buffer.snapshot(),
+                    old_excerpt.range.clone(),
+                    old_excerpt.has_trailing_newline,
+                );
+            } else {
+                new_excerpt = old_excerpt.clone();
+                new_excerpt.buffer = buffer.snapshot();
+            }
+
+            new_excerpts.push(new_excerpt, &());
+            cursor.next(&());
+        }
+        new_excerpts.append(cursor.suffix(&()), &());
+
+        drop(cursor);
+        snapshot.excerpts = new_excerpts;
+
+        self.subscriptions.publish(edits);
+    }
+}
+
+#[cfg(any(test, feature = "test-support"))]
+impl MultiBuffer {
+    pub fn build_simple(text: &str, cx: &mut gpui2::AppContext) -> Model<Self> {
+        let buffer = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), text));
+        cx.build_model(|cx| Self::singleton(buffer, cx))
+    }
+
+    pub fn build_multi<const COUNT: usize>(
+        excerpts: [(&str, Vec<Range<Point>>); COUNT],
+        cx: &mut gpui2::AppContext,
+    ) -> Model<Self> {
+        let multi = cx.build_model(|_| Self::new(0));
+        for (text, ranges) in excerpts {
+            let buffer = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), text));
+            let excerpt_ranges = ranges.into_iter().map(|range| ExcerptRange {
+                context: range,
+                primary: None,
+            });
+            multi.update(cx, |multi, cx| {
+                multi.push_excerpts(buffer, excerpt_ranges, cx)
+            });
+        }
+
+        multi
+    }
+
+    pub fn build_from_buffer(buffer: Model<Buffer>, cx: &mut gpui2::AppContext) -> Model<Self> {
+        cx.build_model(|cx| Self::singleton(buffer, cx))
+    }
+
+    pub fn build_random(rng: &mut impl rand::Rng, cx: &mut gpui2::AppContext) -> Model<Self> {
+        cx.build_model(|cx| {
+            let mut multibuffer = MultiBuffer::new(0);
+            let mutation_count = rng.gen_range(1..=5);
+            multibuffer.randomly_edit_excerpts(rng, mutation_count, cx);
+            multibuffer
+        })
+    }
+
+    pub fn randomly_edit(
+        &mut self,
+        rng: &mut impl rand::Rng,
+        edit_count: usize,
+        cx: &mut ModelContext<Self>,
+    ) {
+        use util::RandomCharIter;
+
+        let snapshot = self.read(cx);
+        let mut edits: Vec<(Range<usize>, Arc<str>)> = Vec::new();
+        let mut last_end = None;
+        for _ in 0..edit_count {
+            if last_end.map_or(false, |last_end| last_end >= snapshot.len()) {
+                break;
+            }
+
+            let new_start = last_end.map_or(0, |last_end| last_end + 1);
+            let end = snapshot.clip_offset(rng.gen_range(new_start..=snapshot.len()), Bias::Right);
+            let start = snapshot.clip_offset(rng.gen_range(new_start..=end), Bias::Right);
+            last_end = Some(end);
+
+            let mut range = start..end;
+            if rng.gen_bool(0.2) {
+                mem::swap(&mut range.start, &mut range.end);
+            }
+
+            let new_text_len = rng.gen_range(0..10);
+            let new_text: String = RandomCharIter::new(&mut *rng).take(new_text_len).collect();
+
+            edits.push((range, new_text.into()));
+        }
+        log::info!("mutating multi-buffer with {:?}", edits);
+        drop(snapshot);
+
+        self.edit(edits, None, cx);
+    }
+
+    pub fn randomly_edit_excerpts(
+        &mut self,
+        rng: &mut impl rand::Rng,
+        mutation_count: usize,
+        cx: &mut ModelContext<Self>,
+    ) {
+        use rand::prelude::*;
+        use std::env;
+        use util::RandomCharIter;
+
+        let max_excerpts = env::var("MAX_EXCERPTS")
+            .map(|i| i.parse().expect("invalid `MAX_EXCERPTS` variable"))
+            .unwrap_or(5);
+
+        let mut buffers = Vec::new();
+        for _ in 0..mutation_count {
+            if rng.gen_bool(0.05) {
+                log::info!("Clearing multi-buffer");
+                self.clear(cx);
+                continue;
+            }
+
+            let excerpt_ids = self.excerpt_ids();
+            if excerpt_ids.is_empty() || (rng.gen() && excerpt_ids.len() < max_excerpts) {
+                let buffer_handle = if rng.gen() || self.buffers.borrow().is_empty() {
+                    let text = RandomCharIter::new(&mut *rng).take(10).collect::<String>();
+                    buffers
+                        .push(cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), text)));
+                    let buffer = buffers.last().unwrap().read(cx);
+                    log::info!(
+                        "Creating new buffer {} with text: {:?}",
+                        buffer.remote_id(),
+                        buffer.text()
+                    );
+                    buffers.last().unwrap().clone()
+                } else {
+                    self.buffers
+                        .borrow()
+                        .values()
+                        .choose(rng)
+                        .unwrap()
+                        .buffer
+                        .clone()
+                };
+
+                let buffer = buffer_handle.read(cx);
+                let buffer_text = buffer.text();
+                let ranges = (0..rng.gen_range(0..5))
+                    .map(|_| {
+                        let end_ix =
+                            buffer.clip_offset(rng.gen_range(0..=buffer.len()), Bias::Right);
+                        let start_ix = buffer.clip_offset(rng.gen_range(0..=end_ix), Bias::Left);
+                        ExcerptRange {
+                            context: start_ix..end_ix,
+                            primary: None,
+                        }
+                    })
+                    .collect::<Vec<_>>();
+                log::info!(
+                    "Inserting excerpts from buffer {} and ranges {:?}: {:?}",
+                    buffer_handle.read(cx).remote_id(),
+                    ranges.iter().map(|r| &r.context).collect::<Vec<_>>(),
+                    ranges
+                        .iter()
+                        .map(|r| &buffer_text[r.context.clone()])
+                        .collect::<Vec<_>>()
+                );
+
+                let excerpt_id = self.push_excerpts(buffer_handle.clone(), ranges, cx);
+                log::info!("Inserted with ids: {:?}", excerpt_id);
+            } else {
+                let remove_count = rng.gen_range(1..=excerpt_ids.len());
+                let mut excerpts_to_remove = excerpt_ids
+                    .choose_multiple(rng, remove_count)
+                    .cloned()
+                    .collect::<Vec<_>>();
+                let snapshot = self.snapshot.borrow();
+                excerpts_to_remove.sort_unstable_by(|a, b| a.cmp(b, &*snapshot));
+                drop(snapshot);
+                log::info!("Removing excerpts {:?}", excerpts_to_remove);
+                self.remove_excerpts(excerpts_to_remove, cx);
+            }
+        }
+    }
+
+    pub fn randomly_mutate(
+        &mut self,
+        rng: &mut impl rand::Rng,
+        mutation_count: usize,
+        cx: &mut ModelContext<Self>,
+    ) {
+        use rand::prelude::*;
+
+        if rng.gen_bool(0.7) || self.singleton {
+            let buffer = self
+                .buffers
+                .borrow()
+                .values()
+                .choose(rng)
+                .map(|state| state.buffer.clone());
+
+            if let Some(buffer) = buffer {
+                buffer.update(cx, |buffer, cx| {
+                    if rng.gen() {
+                        buffer.randomly_edit(rng, mutation_count, cx);
+                    } else {
+                        buffer.randomly_undo_redo(rng, cx);
+                    }
+                });
+            } else {
+                self.randomly_edit(rng, mutation_count, cx);
+            }
+        } else {
+            self.randomly_edit_excerpts(rng, mutation_count, cx);
+        }
+
+        self.check_invariants(cx);
+    }
+
+    fn check_invariants(&self, cx: &mut ModelContext<Self>) {
+        let snapshot = self.read(cx);
+        let excerpts = snapshot.excerpts.items(&());
+        let excerpt_ids = snapshot.excerpt_ids.items(&());
+
+        for (ix, excerpt) in excerpts.iter().enumerate() {
+            if ix == 0 {
+                if excerpt.locator <= Locator::min() {
+                    panic!("invalid first excerpt locator {:?}", excerpt.locator);
+                }
+            } else {
+                if excerpt.locator <= excerpts[ix - 1].locator {
+                    panic!("excerpts are out-of-order: {:?}", excerpts);
+                }
+            }
+        }
+
+        for (ix, entry) in excerpt_ids.iter().enumerate() {
+            if ix == 0 {
+                if entry.id.cmp(&ExcerptId::min(), &*snapshot).is_le() {
+                    panic!("invalid first excerpt id {:?}", entry.id);
+                }
+            } else {
+                if entry.id <= excerpt_ids[ix - 1].id {
+                    panic!("excerpt ids are out-of-order: {:?}", excerpt_ids);
+                }
+            }
+        }
+    }
+}
+
+impl EventEmitter for MultiBuffer {
+    type Event = Event;
+}
+
+impl MultiBufferSnapshot {
+    pub fn text(&self) -> String {
+        self.chunks(0..self.len(), false)
+            .map(|chunk| chunk.text)
+            .collect()
+    }
+
+    pub fn reversed_chars_at<T: ToOffset>(&self, position: T) -> impl Iterator<Item = char> + '_ {
+        let mut offset = position.to_offset(self);
+        let mut cursor = self.excerpts.cursor::<usize>();
+        cursor.seek(&offset, Bias::Left, &());
+        let mut excerpt_chunks = cursor.item().map(|excerpt| {
+            let end_before_footer = cursor.start() + excerpt.text_summary.len;
+            let start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let end = start + (cmp::min(offset, end_before_footer) - cursor.start());
+            excerpt.buffer.reversed_chunks_in_range(start..end)
+        });
+        iter::from_fn(move || {
+            if offset == *cursor.start() {
+                cursor.prev(&());
+                let excerpt = cursor.item()?;
+                excerpt_chunks = Some(
+                    excerpt
+                        .buffer
+                        .reversed_chunks_in_range(excerpt.range.context.clone()),
+                );
+            }
+
+            let excerpt = cursor.item().unwrap();
+            if offset == cursor.end(&()) && excerpt.has_trailing_newline {
+                offset -= 1;
+                Some("\n")
+            } else {
+                let chunk = excerpt_chunks.as_mut().unwrap().next().unwrap();
+                offset -= chunk.len();
+                Some(chunk)
+            }
+        })
+        .flat_map(|c| c.chars().rev())
+    }
+
+    pub fn chars_at<T: ToOffset>(&self, position: T) -> impl Iterator<Item = char> + '_ {
+        let offset = position.to_offset(self);
+        self.text_for_range(offset..self.len())
+            .flat_map(|chunk| chunk.chars())
+    }
+
+    pub fn text_for_range<T: ToOffset>(&self, range: Range<T>) -> impl Iterator<Item = &str> + '_ {
+        self.chunks(range, false).map(|chunk| chunk.text)
+    }
+
+    pub fn is_line_blank(&self, row: u32) -> bool {
+        self.text_for_range(Point::new(row, 0)..Point::new(row, self.line_len(row)))
+            .all(|chunk| chunk.matches(|c: char| !c.is_whitespace()).next().is_none())
+    }
+
+    pub fn contains_str_at<T>(&self, position: T, needle: &str) -> bool
+    where
+        T: ToOffset,
+    {
+        let position = position.to_offset(self);
+        position == self.clip_offset(position, Bias::Left)
+            && self
+                .bytes_in_range(position..self.len())
+                .flatten()
+                .copied()
+                .take(needle.len())
+                .eq(needle.bytes())
+    }
+
+    pub fn surrounding_word<T: ToOffset>(&self, start: T) -> (Range<usize>, Option<CharKind>) {
+        let mut start = start.to_offset(self);
+        let mut end = start;
+        let mut next_chars = self.chars_at(start).peekable();
+        let mut prev_chars = self.reversed_chars_at(start).peekable();
+
+        let scope = self.language_scope_at(start);
+        let kind = |c| char_kind(&scope, c);
+        let word_kind = cmp::max(
+            prev_chars.peek().copied().map(kind),
+            next_chars.peek().copied().map(kind),
+        );
+
+        for ch in prev_chars {
+            if Some(kind(ch)) == word_kind && ch != '\n' {
+                start -= ch.len_utf8();
+            } else {
+                break;
+            }
+        }
+
+        for ch in next_chars {
+            if Some(kind(ch)) == word_kind && ch != '\n' {
+                end += ch.len_utf8();
+            } else {
+                break;
+            }
+        }
+
+        (start..end, word_kind)
+    }
+
+    pub fn as_singleton(&self) -> Option<(&ExcerptId, u64, &BufferSnapshot)> {
+        if self.singleton {
+            self.excerpts
+                .iter()
+                .next()
+                .map(|e| (&e.id, e.buffer_id, &e.buffer))
+        } else {
+            None
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.excerpts.summary().text.len
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.excerpts.summary().text.len == 0
+    }
+
+    pub fn max_buffer_row(&self) -> u32 {
+        self.excerpts.summary().max_buffer_row
+    }
+
+    pub fn clip_offset(&self, offset: usize, bias: Bias) -> usize {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.clip_offset(offset, bias);
+        }
+
+        let mut cursor = self.excerpts.cursor::<usize>();
+        cursor.seek(&offset, Bias::Right, &());
+        let overshoot = if let Some(excerpt) = cursor.item() {
+            let excerpt_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let buffer_offset = excerpt
+                .buffer
+                .clip_offset(excerpt_start + (offset - cursor.start()), bias);
+            buffer_offset.saturating_sub(excerpt_start)
+        } else {
+            0
+        };
+        cursor.start() + overshoot
+    }
+
+    pub fn clip_point(&self, point: Point, bias: Bias) -> Point {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.clip_point(point, bias);
+        }
+
+        let mut cursor = self.excerpts.cursor::<Point>();
+        cursor.seek(&point, Bias::Right, &());
+        let overshoot = if let Some(excerpt) = cursor.item() {
+            let excerpt_start = excerpt.range.context.start.to_point(&excerpt.buffer);
+            let buffer_point = excerpt
+                .buffer
+                .clip_point(excerpt_start + (point - cursor.start()), bias);
+            buffer_point.saturating_sub(excerpt_start)
+        } else {
+            Point::zero()
+        };
+        *cursor.start() + overshoot
+    }
+
+    pub fn clip_offset_utf16(&self, offset: OffsetUtf16, bias: Bias) -> OffsetUtf16 {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.clip_offset_utf16(offset, bias);
+        }
+
+        let mut cursor = self.excerpts.cursor::<OffsetUtf16>();
+        cursor.seek(&offset, Bias::Right, &());
+        let overshoot = if let Some(excerpt) = cursor.item() {
+            let excerpt_start = excerpt.range.context.start.to_offset_utf16(&excerpt.buffer);
+            let buffer_offset = excerpt
+                .buffer
+                .clip_offset_utf16(excerpt_start + (offset - cursor.start()), bias);
+            OffsetUtf16(buffer_offset.0.saturating_sub(excerpt_start.0))
+        } else {
+            OffsetUtf16(0)
+        };
+        *cursor.start() + overshoot
+    }
+
+    pub fn clip_point_utf16(&self, point: Unclipped<PointUtf16>, bias: Bias) -> PointUtf16 {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.clip_point_utf16(point, bias);
+        }
+
+        let mut cursor = self.excerpts.cursor::<PointUtf16>();
+        cursor.seek(&point.0, Bias::Right, &());
+        let overshoot = if let Some(excerpt) = cursor.item() {
+            let excerpt_start = excerpt
+                .buffer
+                .offset_to_point_utf16(excerpt.range.context.start.to_offset(&excerpt.buffer));
+            let buffer_point = excerpt
+                .buffer
+                .clip_point_utf16(Unclipped(excerpt_start + (point.0 - cursor.start())), bias);
+            buffer_point.saturating_sub(excerpt_start)
+        } else {
+            PointUtf16::zero()
+        };
+        *cursor.start() + overshoot
+    }
+
+    pub fn bytes_in_range<T: ToOffset>(&self, range: Range<T>) -> MultiBufferBytes {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+        let mut excerpts = self.excerpts.cursor::<usize>();
+        excerpts.seek(&range.start, Bias::Right, &());
+
+        let mut chunk = &[][..];
+        let excerpt_bytes = if let Some(excerpt) = excerpts.item() {
+            let mut excerpt_bytes = excerpt
+                .bytes_in_range(range.start - excerpts.start()..range.end - excerpts.start());
+            chunk = excerpt_bytes.next().unwrap_or(&[][..]);
+            Some(excerpt_bytes)
+        } else {
+            None
+        };
+        MultiBufferBytes {
+            range,
+            excerpts,
+            excerpt_bytes,
+            chunk,
+        }
+    }
+
+    pub fn reversed_bytes_in_range<T: ToOffset>(
+        &self,
+        range: Range<T>,
+    ) -> ReversedMultiBufferBytes {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+        let mut excerpts = self.excerpts.cursor::<usize>();
+        excerpts.seek(&range.end, Bias::Left, &());
+
+        let mut chunk = &[][..];
+        let excerpt_bytes = if let Some(excerpt) = excerpts.item() {
+            let mut excerpt_bytes = excerpt.reversed_bytes_in_range(
+                range.start - excerpts.start()..range.end - excerpts.start(),
+            );
+            chunk = excerpt_bytes.next().unwrap_or(&[][..]);
+            Some(excerpt_bytes)
+        } else {
+            None
+        };
+
+        ReversedMultiBufferBytes {
+            range,
+            excerpts,
+            excerpt_bytes,
+            chunk,
+        }
+    }
+
+    pub fn buffer_rows(&self, start_row: u32) -> MultiBufferRows {
+        let mut result = MultiBufferRows {
+            buffer_row_range: 0..0,
+            excerpts: self.excerpts.cursor(),
+        };
+        result.seek(start_row);
+        result
+    }
+
+    pub fn chunks<T: ToOffset>(&self, range: Range<T>, language_aware: bool) -> MultiBufferChunks {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+        let mut chunks = MultiBufferChunks {
+            range: range.clone(),
+            excerpts: self.excerpts.cursor(),
+            excerpt_chunks: None,
+            language_aware,
+        };
+        chunks.seek(range.start);
+        chunks
+    }
+
+    pub fn offset_to_point(&self, offset: usize) -> Point {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.offset_to_point(offset);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(usize, Point)>();
+        cursor.seek(&offset, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_offset, start_point) = cursor.start();
+            let overshoot = offset - start_offset;
+            let excerpt_start_offset = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let excerpt_start_point = excerpt.range.context.start.to_point(&excerpt.buffer);
+            let buffer_point = excerpt
+                .buffer
+                .offset_to_point(excerpt_start_offset + overshoot);
+            *start_point + (buffer_point - excerpt_start_point)
+        } else {
+            self.excerpts.summary().text.lines
+        }
+    }
+
+    pub fn offset_to_point_utf16(&self, offset: usize) -> PointUtf16 {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.offset_to_point_utf16(offset);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(usize, PointUtf16)>();
+        cursor.seek(&offset, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_offset, start_point) = cursor.start();
+            let overshoot = offset - start_offset;
+            let excerpt_start_offset = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let excerpt_start_point = excerpt.range.context.start.to_point_utf16(&excerpt.buffer);
+            let buffer_point = excerpt
+                .buffer
+                .offset_to_point_utf16(excerpt_start_offset + overshoot);
+            *start_point + (buffer_point - excerpt_start_point)
+        } else {
+            self.excerpts.summary().text.lines_utf16()
+        }
+    }
+
+    pub fn point_to_point_utf16(&self, point: Point) -> PointUtf16 {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.point_to_point_utf16(point);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(Point, PointUtf16)>();
+        cursor.seek(&point, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_offset, start_point) = cursor.start();
+            let overshoot = point - start_offset;
+            let excerpt_start_point = excerpt.range.context.start.to_point(&excerpt.buffer);
+            let excerpt_start_point_utf16 =
+                excerpt.range.context.start.to_point_utf16(&excerpt.buffer);
+            let buffer_point = excerpt
+                .buffer
+                .point_to_point_utf16(excerpt_start_point + overshoot);
+            *start_point + (buffer_point - excerpt_start_point_utf16)
+        } else {
+            self.excerpts.summary().text.lines_utf16()
+        }
+    }
+
+    pub fn point_to_offset(&self, point: Point) -> usize {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.point_to_offset(point);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(Point, usize)>();
+        cursor.seek(&point, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_point, start_offset) = cursor.start();
+            let overshoot = point - start_point;
+            let excerpt_start_offset = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let excerpt_start_point = excerpt.range.context.start.to_point(&excerpt.buffer);
+            let buffer_offset = excerpt
+                .buffer
+                .point_to_offset(excerpt_start_point + overshoot);
+            *start_offset + buffer_offset - excerpt_start_offset
+        } else {
+            self.excerpts.summary().text.len
+        }
+    }
+
+    pub fn offset_utf16_to_offset(&self, offset_utf16: OffsetUtf16) -> usize {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.offset_utf16_to_offset(offset_utf16);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(OffsetUtf16, usize)>();
+        cursor.seek(&offset_utf16, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_offset_utf16, start_offset) = cursor.start();
+            let overshoot = offset_utf16 - start_offset_utf16;
+            let excerpt_start_offset = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let excerpt_start_offset_utf16 =
+                excerpt.buffer.offset_to_offset_utf16(excerpt_start_offset);
+            let buffer_offset = excerpt
+                .buffer
+                .offset_utf16_to_offset(excerpt_start_offset_utf16 + overshoot);
+            *start_offset + (buffer_offset - excerpt_start_offset)
+        } else {
+            self.excerpts.summary().text.len
+        }
+    }
+
+    pub fn offset_to_offset_utf16(&self, offset: usize) -> OffsetUtf16 {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.offset_to_offset_utf16(offset);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(usize, OffsetUtf16)>();
+        cursor.seek(&offset, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_offset, start_offset_utf16) = cursor.start();
+            let overshoot = offset - start_offset;
+            let excerpt_start_offset_utf16 =
+                excerpt.range.context.start.to_offset_utf16(&excerpt.buffer);
+            let excerpt_start_offset = excerpt
+                .buffer
+                .offset_utf16_to_offset(excerpt_start_offset_utf16);
+            let buffer_offset_utf16 = excerpt
+                .buffer
+                .offset_to_offset_utf16(excerpt_start_offset + overshoot);
+            *start_offset_utf16 + (buffer_offset_utf16 - excerpt_start_offset_utf16)
+        } else {
+            self.excerpts.summary().text.len_utf16
+        }
+    }
+
+    pub fn point_utf16_to_offset(&self, point: PointUtf16) -> usize {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer.point_utf16_to_offset(point);
+        }
+
+        let mut cursor = self.excerpts.cursor::<(PointUtf16, usize)>();
+        cursor.seek(&point, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let (start_point, start_offset) = cursor.start();
+            let overshoot = point - start_point;
+            let excerpt_start_offset = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let excerpt_start_point = excerpt
+                .buffer
+                .offset_to_point_utf16(excerpt.range.context.start.to_offset(&excerpt.buffer));
+            let buffer_offset = excerpt
+                .buffer
+                .point_utf16_to_offset(excerpt_start_point + overshoot);
+            *start_offset + (buffer_offset - excerpt_start_offset)
+        } else {
+            self.excerpts.summary().text.len
+        }
+    }
+
+    pub fn point_to_buffer_offset<T: ToOffset>(
+        &self,
+        point: T,
+    ) -> Option<(&BufferSnapshot, usize)> {
+        let offset = point.to_offset(&self);
+        let mut cursor = self.excerpts.cursor::<usize>();
+        cursor.seek(&offset, Bias::Right, &());
+        if cursor.item().is_none() {
+            cursor.prev(&());
+        }
+
+        cursor.item().map(|excerpt| {
+            let excerpt_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let buffer_point = excerpt_start + offset - *cursor.start();
+            (&excerpt.buffer, buffer_point)
+        })
+    }
+
+    pub fn suggested_indents(
+        &self,
+        rows: impl IntoIterator<Item = u32>,
+        cx: &AppContext,
+    ) -> BTreeMap<u32, IndentSize> {
+        let mut result = BTreeMap::new();
+
+        let mut rows_for_excerpt = Vec::new();
+        let mut cursor = self.excerpts.cursor::<Point>();
+        let mut rows = rows.into_iter().peekable();
+        let mut prev_row = u32::MAX;
+        let mut prev_language_indent_size = IndentSize::default();
+
+        while let Some(row) = rows.next() {
+            cursor.seek(&Point::new(row, 0), Bias::Right, &());
+            let excerpt = match cursor.item() {
+                Some(excerpt) => excerpt,
+                _ => continue,
+            };
+
+            // Retrieve the language and indent size once for each disjoint region being indented.
+            let single_indent_size = if row.saturating_sub(1) == prev_row {
+                prev_language_indent_size
+            } else {
+                excerpt
+                    .buffer
+                    .language_indent_size_at(Point::new(row, 0), cx)
+            };
+            prev_language_indent_size = single_indent_size;
+            prev_row = row;
+
+            let start_buffer_row = excerpt.range.context.start.to_point(&excerpt.buffer).row;
+            let start_multibuffer_row = cursor.start().row;
+
+            rows_for_excerpt.push(row);
+            while let Some(next_row) = rows.peek().copied() {
+                if cursor.end(&()).row > next_row {
+                    rows_for_excerpt.push(next_row);
+                    rows.next();
+                } else {
+                    break;
+                }
+            }
+
+            let buffer_rows = rows_for_excerpt
+                .drain(..)
+                .map(|row| start_buffer_row + row - start_multibuffer_row);
+            let buffer_indents = excerpt
+                .buffer
+                .suggested_indents(buffer_rows, single_indent_size);
+            let multibuffer_indents = buffer_indents
+                .into_iter()
+                .map(|(row, indent)| (start_multibuffer_row + row - start_buffer_row, indent));
+            result.extend(multibuffer_indents);
+        }
+
+        result
+    }
+
+    pub fn indent_size_for_line(&self, row: u32) -> IndentSize {
+        if let Some((buffer, range)) = self.buffer_line_for_row(row) {
+            let mut size = buffer.indent_size_for_line(range.start.row);
+            size.len = size
+                .len
+                .min(range.end.column)
+                .saturating_sub(range.start.column);
+            size
+        } else {
+            IndentSize::spaces(0)
+        }
+    }
+
+    pub fn prev_non_blank_row(&self, mut row: u32) -> Option<u32> {
+        while row > 0 {
+            row -= 1;
+            if !self.is_line_blank(row) {
+                return Some(row);
+            }
+        }
+        None
+    }
+
+    pub fn line_len(&self, row: u32) -> u32 {
+        if let Some((_, range)) = self.buffer_line_for_row(row) {
+            range.end.column - range.start.column
+        } else {
+            0
+        }
+    }
+
+    pub fn buffer_line_for_row(&self, row: u32) -> Option<(&BufferSnapshot, Range<Point>)> {
+        let mut cursor = self.excerpts.cursor::<Point>();
+        let point = Point::new(row, 0);
+        cursor.seek(&point, Bias::Right, &());
+        if cursor.item().is_none() && *cursor.start() == point {
+            cursor.prev(&());
+        }
+        if let Some(excerpt) = cursor.item() {
+            let overshoot = row - cursor.start().row;
+            let excerpt_start = excerpt.range.context.start.to_point(&excerpt.buffer);
+            let excerpt_end = excerpt.range.context.end.to_point(&excerpt.buffer);
+            let buffer_row = excerpt_start.row + overshoot;
+            let line_start = Point::new(buffer_row, 0);
+            let line_end = Point::new(buffer_row, excerpt.buffer.line_len(buffer_row));
+            return Some((
+                &excerpt.buffer,
+                line_start.max(excerpt_start)..line_end.min(excerpt_end),
+            ));
+        }
+        None
+    }
+
+    pub fn max_point(&self) -> Point {
+        self.text_summary().lines
+    }
+
+    pub fn text_summary(&self) -> TextSummary {
+        self.excerpts.summary().text.clone()
+    }
+
+    pub fn text_summary_for_range<D, O>(&self, range: Range<O>) -> D
+    where
+        D: TextDimension,
+        O: ToOffset,
+    {
+        let mut summary = D::default();
+        let mut range = range.start.to_offset(self)..range.end.to_offset(self);
+        let mut cursor = self.excerpts.cursor::<usize>();
+        cursor.seek(&range.start, Bias::Right, &());
+        if let Some(excerpt) = cursor.item() {
+            let mut end_before_newline = cursor.end(&());
+            if excerpt.has_trailing_newline {
+                end_before_newline -= 1;
+            }
+
+            let excerpt_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let start_in_excerpt = excerpt_start + (range.start - cursor.start());
+            let end_in_excerpt =
+                excerpt_start + (cmp::min(end_before_newline, range.end) - cursor.start());
+            summary.add_assign(
+                &excerpt
+                    .buffer
+                    .text_summary_for_range(start_in_excerpt..end_in_excerpt),
+            );
+
+            if range.end > end_before_newline {
+                summary.add_assign(&D::from_text_summary(&TextSummary::from("\n")));
+            }
+
+            cursor.next(&());
+        }
+
+        if range.end > *cursor.start() {
+            summary.add_assign(&D::from_text_summary(&cursor.summary::<_, TextSummary>(
+                &range.end,
+                Bias::Right,
+                &(),
+            )));
+            if let Some(excerpt) = cursor.item() {
+                range.end = cmp::max(*cursor.start(), range.end);
+
+                let excerpt_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+                let end_in_excerpt = excerpt_start + (range.end - cursor.start());
+                summary.add_assign(
+                    &excerpt
+                        .buffer
+                        .text_summary_for_range(excerpt_start..end_in_excerpt),
+                );
+            }
+        }
+
+        summary
+    }
+
+    pub fn summary_for_anchor<D>(&self, anchor: &Anchor) -> D
+    where
+        D: TextDimension + Ord + Sub<D, Output = D>,
+    {
+        let mut cursor = self.excerpts.cursor::<ExcerptSummary>();
+        let locator = self.excerpt_locator_for_id(anchor.excerpt_id);
+
+        cursor.seek(locator, Bias::Left, &());
+        if cursor.item().is_none() {
+            cursor.next(&());
+        }
+
+        let mut position = D::from_text_summary(&cursor.start().text);
+        if let Some(excerpt) = cursor.item() {
+            if excerpt.id == anchor.excerpt_id {
+                let excerpt_buffer_start =
+                    excerpt.range.context.start.summary::<D>(&excerpt.buffer);
+                let excerpt_buffer_end = excerpt.range.context.end.summary::<D>(&excerpt.buffer);
+                let buffer_position = cmp::min(
+                    excerpt_buffer_end,
+                    anchor.text_anchor.summary::<D>(&excerpt.buffer),
+                );
+                if buffer_position > excerpt_buffer_start {
+                    position.add_assign(&(buffer_position - excerpt_buffer_start));
+                }
+            }
+        }
+        position
+    }
+
+    pub fn summaries_for_anchors<'a, D, I>(&'a self, anchors: I) -> Vec<D>
+    where
+        D: TextDimension + Ord + Sub<D, Output = D>,
+        I: 'a + IntoIterator<Item = &'a Anchor>,
+    {
+        if let Some((_, _, buffer)) = self.as_singleton() {
+            return buffer
+                .summaries_for_anchors(anchors.into_iter().map(|a| &a.text_anchor))
+                .collect();
+        }
+
+        let mut anchors = anchors.into_iter().peekable();
+        let mut cursor = self.excerpts.cursor::<ExcerptSummary>();
+        let mut summaries = Vec::new();
+        while let Some(anchor) = anchors.peek() {
+            let excerpt_id = anchor.excerpt_id;
+            let excerpt_anchors = iter::from_fn(|| {
+                let anchor = anchors.peek()?;
+                if anchor.excerpt_id == excerpt_id {
+                    Some(&anchors.next().unwrap().text_anchor)
+                } else {
+                    None
+                }
+            });
+
+            let locator = self.excerpt_locator_for_id(excerpt_id);
+            cursor.seek_forward(locator, Bias::Left, &());
+            if cursor.item().is_none() {
+                cursor.next(&());
+            }
+
+            let position = D::from_text_summary(&cursor.start().text);
+            if let Some(excerpt) = cursor.item() {
+                if excerpt.id == excerpt_id {
+                    let excerpt_buffer_start =
+                        excerpt.range.context.start.summary::<D>(&excerpt.buffer);
+                    let excerpt_buffer_end =
+                        excerpt.range.context.end.summary::<D>(&excerpt.buffer);
+                    summaries.extend(
+                        excerpt
+                            .buffer
+                            .summaries_for_anchors::<D, _>(excerpt_anchors)
+                            .map(move |summary| {
+                                let summary = cmp::min(excerpt_buffer_end.clone(), summary);
+                                let mut position = position.clone();
+                                let excerpt_buffer_start = excerpt_buffer_start.clone();
+                                if summary > excerpt_buffer_start {
+                                    position.add_assign(&(summary - excerpt_buffer_start));
+                                }
+                                position
+                            }),
+                    );
+                    continue;
+                }
+            }
+
+            summaries.extend(excerpt_anchors.map(|_| position.clone()));
+        }
+
+        summaries
+    }
+
+    pub fn refresh_anchors<'a, I>(&'a self, anchors: I) -> Vec<(usize, Anchor, bool)>
+    where
+        I: 'a + IntoIterator<Item = &'a Anchor>,
+    {
+        let mut anchors = anchors.into_iter().enumerate().peekable();
+        let mut cursor = self.excerpts.cursor::<Option<&Locator>>();
+        cursor.next(&());
+
+        let mut result = Vec::new();
+
+        while let Some((_, anchor)) = anchors.peek() {
+            let old_excerpt_id = anchor.excerpt_id;
+
+            // Find the location where this anchor's excerpt should be.
+            let old_locator = self.excerpt_locator_for_id(old_excerpt_id);
+            cursor.seek_forward(&Some(old_locator), Bias::Left, &());
+
+            if cursor.item().is_none() {
+                cursor.next(&());
+            }
+
+            let next_excerpt = cursor.item();
+            let prev_excerpt = cursor.prev_item();
+
+            // Process all of the anchors for this excerpt.
+            while let Some((_, anchor)) = anchors.peek() {
+                if anchor.excerpt_id != old_excerpt_id {
+                    break;
+                }
+                let (anchor_ix, anchor) = anchors.next().unwrap();
+                let mut anchor = *anchor;
+
+                // Leave min and max anchors unchanged if invalid or
+                // if the old excerpt still exists at this location
+                let mut kept_position = next_excerpt
+                    .map_or(false, |e| e.id == old_excerpt_id && e.contains(&anchor))
+                    || old_excerpt_id == ExcerptId::max()
+                    || old_excerpt_id == ExcerptId::min();
+
+                // If the old excerpt no longer exists at this location, then attempt to
+                // find an equivalent position for this anchor in an adjacent excerpt.
+                if !kept_position {
+                    for excerpt in [next_excerpt, prev_excerpt].iter().filter_map(|e| *e) {
+                        if excerpt.contains(&anchor) {
+                            anchor.excerpt_id = excerpt.id.clone();
+                            kept_position = true;
+                            break;
+                        }
+                    }
+                }
+
+                // If there's no adjacent excerpt that contains the anchor's position,
+                // then report that the anchor has lost its position.
+                if !kept_position {
+                    anchor = if let Some(excerpt) = next_excerpt {
+                        let mut text_anchor = excerpt
+                            .range
+                            .context
+                            .start
+                            .bias(anchor.text_anchor.bias, &excerpt.buffer);
+                        if text_anchor
+                            .cmp(&excerpt.range.context.end, &excerpt.buffer)
+                            .is_gt()
+                        {
+                            text_anchor = excerpt.range.context.end;
+                        }
+                        Anchor {
+                            buffer_id: Some(excerpt.buffer_id),
+                            excerpt_id: excerpt.id.clone(),
+                            text_anchor,
+                        }
+                    } else if let Some(excerpt) = prev_excerpt {
+                        let mut text_anchor = excerpt
+                            .range
+                            .context
+                            .end
+                            .bias(anchor.text_anchor.bias, &excerpt.buffer);
+                        if text_anchor
+                            .cmp(&excerpt.range.context.start, &excerpt.buffer)
+                            .is_lt()
+                        {
+                            text_anchor = excerpt.range.context.start;
+                        }
+                        Anchor {
+                            buffer_id: Some(excerpt.buffer_id),
+                            excerpt_id: excerpt.id.clone(),
+                            text_anchor,
+                        }
+                    } else if anchor.text_anchor.bias == Bias::Left {
+                        Anchor::min()
+                    } else {
+                        Anchor::max()
+                    };
+                }
+
+                result.push((anchor_ix, anchor, kept_position));
+            }
+        }
+        result.sort_unstable_by(|a, b| a.1.cmp(&b.1, self));
+        result
+    }
+
+    pub fn anchor_before<T: ToOffset>(&self, position: T) -> Anchor {
+        self.anchor_at(position, Bias::Left)
+    }
+
+    pub fn anchor_after<T: ToOffset>(&self, position: T) -> Anchor {
+        self.anchor_at(position, Bias::Right)
+    }
+
+    pub fn anchor_at<T: ToOffset>(&self, position: T, mut bias: Bias) -> Anchor {
+        let offset = position.to_offset(self);
+        if let Some((excerpt_id, buffer_id, buffer)) = self.as_singleton() {
+            return Anchor {
+                buffer_id: Some(buffer_id),
+                excerpt_id: excerpt_id.clone(),
+                text_anchor: buffer.anchor_at(offset, bias),
+            };
+        }
+
+        let mut cursor = self.excerpts.cursor::<(usize, Option<ExcerptId>)>();
+        cursor.seek(&offset, Bias::Right, &());
+        if cursor.item().is_none() && offset == cursor.start().0 && bias == Bias::Left {
+            cursor.prev(&());
+        }
+        if let Some(excerpt) = cursor.item() {
+            let mut overshoot = offset.saturating_sub(cursor.start().0);
+            if excerpt.has_trailing_newline && offset == cursor.end(&()).0 {
+                overshoot -= 1;
+                bias = Bias::Right;
+            }
+
+            let buffer_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let text_anchor =
+                excerpt.clip_anchor(excerpt.buffer.anchor_at(buffer_start + overshoot, bias));
+            Anchor {
+                buffer_id: Some(excerpt.buffer_id),
+                excerpt_id: excerpt.id.clone(),
+                text_anchor,
+            }
+        } else if offset == 0 && bias == Bias::Left {
+            Anchor::min()
+        } else {
+            Anchor::max()
+        }
+    }
+
+    pub fn anchor_in_excerpt(&self, excerpt_id: ExcerptId, text_anchor: text::Anchor) -> Anchor {
+        let locator = self.excerpt_locator_for_id(excerpt_id);
+        let mut cursor = self.excerpts.cursor::<Option<&Locator>>();
+        cursor.seek(locator, Bias::Left, &());
+        if let Some(excerpt) = cursor.item() {
+            if excerpt.id == excerpt_id {
+                let text_anchor = excerpt.clip_anchor(text_anchor);
+                drop(cursor);
+                return Anchor {
+                    buffer_id: Some(excerpt.buffer_id),
+                    excerpt_id,
+                    text_anchor,
+                };
+            }
+        }
+        panic!("excerpt not found");
+    }
+
+    pub fn can_resolve(&self, anchor: &Anchor) -> bool {
+        if anchor.excerpt_id == ExcerptId::min() || anchor.excerpt_id == ExcerptId::max() {
+            true
+        } else if let Some(excerpt) = self.excerpt(anchor.excerpt_id) {
+            excerpt.buffer.can_resolve(&anchor.text_anchor)
+        } else {
+            false
+        }
+    }
+
+    pub fn excerpts(
+        &self,
+    ) -> impl Iterator<Item = (ExcerptId, &BufferSnapshot, ExcerptRange<text::Anchor>)> {
+        self.excerpts
+            .iter()
+            .map(|excerpt| (excerpt.id, &excerpt.buffer, excerpt.range.clone()))
+    }
+
+    pub fn excerpt_boundaries_in_range<R, T>(
+        &self,
+        range: R,
+    ) -> impl Iterator<Item = ExcerptBoundary> + '_
+    where
+        R: RangeBounds<T>,
+        T: ToOffset,
+    {
+        let start_offset;
+        let start = match range.start_bound() {
+            Bound::Included(start) => {
+                start_offset = start.to_offset(self);
+                Bound::Included(start_offset)
+            }
+            Bound::Excluded(start) => {
+                start_offset = start.to_offset(self);
+                Bound::Excluded(start_offset)
+            }
+            Bound::Unbounded => {
+                start_offset = 0;
+                Bound::Unbounded
+            }
+        };
+        let end = match range.end_bound() {
+            Bound::Included(end) => Bound::Included(end.to_offset(self)),
+            Bound::Excluded(end) => Bound::Excluded(end.to_offset(self)),
+            Bound::Unbounded => Bound::Unbounded,
+        };
+        let bounds = (start, end);
+
+        let mut cursor = self.excerpts.cursor::<(usize, Point)>();
+        cursor.seek(&start_offset, Bias::Right, &());
+        if cursor.item().is_none() {
+            cursor.prev(&());
+        }
+        if !bounds.contains(&cursor.start().0) {
+            cursor.next(&());
+        }
+
+        let mut prev_buffer_id = cursor.prev_item().map(|excerpt| excerpt.buffer_id);
+        std::iter::from_fn(move || {
+            if self.singleton {
+                None
+            } else if bounds.contains(&cursor.start().0) {
+                let excerpt = cursor.item()?;
+                let starts_new_buffer = Some(excerpt.buffer_id) != prev_buffer_id;
+                let boundary = ExcerptBoundary {
+                    id: excerpt.id.clone(),
+                    row: cursor.start().1.row,
+                    buffer: excerpt.buffer.clone(),
+                    range: excerpt.range.clone(),
+                    starts_new_buffer,
+                };
+
+                prev_buffer_id = Some(excerpt.buffer_id);
+                cursor.next(&());
+                Some(boundary)
+            } else {
+                None
+            }
+        })
+    }
+
+    pub fn edit_count(&self) -> usize {
+        self.edit_count
+    }
+
+    pub fn parse_count(&self) -> usize {
+        self.parse_count
+    }
+
+    /// Returns the smallest enclosing bracket ranges containing the given range or
+    /// None if no brackets contain range or the range is not contained in a single
+    /// excerpt
+    pub fn innermost_enclosing_bracket_ranges<T: ToOffset>(
+        &self,
+        range: Range<T>,
+    ) -> Option<(Range<usize>, Range<usize>)> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+
+        // Get the ranges of the innermost pair of brackets.
+        let mut result: Option<(Range<usize>, Range<usize>)> = None;
+
+        let Some(enclosing_bracket_ranges) = self.enclosing_bracket_ranges(range.clone()) else {
+            return None;
+        };
+
+        for (open, close) in enclosing_bracket_ranges {
+            let len = close.end - open.start;
+
+            if let Some((existing_open, existing_close)) = &result {
+                let existing_len = existing_close.end - existing_open.start;
+                if len > existing_len {
+                    continue;
+                }
+            }
+
+            result = Some((open, close));
+        }
+
+        result
+    }
+
+    /// Returns enclosing bracket ranges containing the given range or returns None if the range is
+    /// not contained in a single excerpt
+    pub fn enclosing_bracket_ranges<'a, T: ToOffset>(
+        &'a self,
+        range: Range<T>,
+    ) -> Option<impl Iterator<Item = (Range<usize>, Range<usize>)> + 'a> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+
+        self.bracket_ranges(range.clone()).map(|range_pairs| {
+            range_pairs
+                .filter(move |(open, close)| open.start <= range.start && close.end >= range.end)
+        })
+    }
+
+    /// Returns bracket range pairs overlapping the given `range` or returns None if the `range` is
+    /// not contained in a single excerpt
+    pub fn bracket_ranges<'a, T: ToOffset>(
+        &'a self,
+        range: Range<T>,
+    ) -> Option<impl Iterator<Item = (Range<usize>, Range<usize>)> + 'a> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+        let excerpt = self.excerpt_containing(range.clone());
+        excerpt.map(|(excerpt, excerpt_offset)| {
+            let excerpt_buffer_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+            let excerpt_buffer_end = excerpt_buffer_start + excerpt.text_summary.len;
+
+            let start_in_buffer = excerpt_buffer_start + range.start.saturating_sub(excerpt_offset);
+            let end_in_buffer = excerpt_buffer_start + range.end.saturating_sub(excerpt_offset);
+
+            excerpt
+                .buffer
+                .bracket_ranges(start_in_buffer..end_in_buffer)
+                .filter_map(move |(start_bracket_range, end_bracket_range)| {
+                    if start_bracket_range.start < excerpt_buffer_start
+                        || end_bracket_range.end > excerpt_buffer_end
+                    {
+                        return None;
+                    }
+
+                    let mut start_bracket_range = start_bracket_range.clone();
+                    start_bracket_range.start =
+                        excerpt_offset + (start_bracket_range.start - excerpt_buffer_start);
+                    start_bracket_range.end =
+                        excerpt_offset + (start_bracket_range.end - excerpt_buffer_start);
+
+                    let mut end_bracket_range = end_bracket_range.clone();
+                    end_bracket_range.start =
+                        excerpt_offset + (end_bracket_range.start - excerpt_buffer_start);
+                    end_bracket_range.end =
+                        excerpt_offset + (end_bracket_range.end - excerpt_buffer_start);
+                    Some((start_bracket_range, end_bracket_range))
+                })
+        })
+    }
+
+    pub fn diagnostics_update_count(&self) -> usize {
+        self.diagnostics_update_count
+    }
+
+    pub fn git_diff_update_count(&self) -> usize {
+        self.git_diff_update_count
+    }
+
+    pub fn trailing_excerpt_update_count(&self) -> usize {
+        self.trailing_excerpt_update_count
+    }
+
+    pub fn file_at<'a, T: ToOffset>(&'a self, point: T) -> Option<&'a Arc<dyn File>> {
+        self.point_to_buffer_offset(point)
+            .and_then(|(buffer, _)| buffer.file())
+    }
+
+    pub fn language_at<'a, T: ToOffset>(&'a self, point: T) -> Option<&'a Arc<Language>> {
+        self.point_to_buffer_offset(point)
+            .and_then(|(buffer, offset)| buffer.language_at(offset))
+    }
+
+    pub fn settings_at<'a, T: ToOffset>(
+        &'a self,
+        point: T,
+        cx: &'a AppContext,
+    ) -> &'a LanguageSettings {
+        let mut language = None;
+        let mut file = None;
+        if let Some((buffer, offset)) = self.point_to_buffer_offset(point) {
+            language = buffer.language_at(offset);
+            file = buffer.file();
+        }
+        language_settings(language, file, cx)
+    }
+
+    pub fn language_scope_at<'a, T: ToOffset>(&'a self, point: T) -> Option<LanguageScope> {
+        self.point_to_buffer_offset(point)
+            .and_then(|(buffer, offset)| buffer.language_scope_at(offset))
+    }
+
+    pub fn language_indent_size_at<T: ToOffset>(
+        &self,
+        position: T,
+        cx: &AppContext,
+    ) -> Option<IndentSize> {
+        let (buffer_snapshot, offset) = self.point_to_buffer_offset(position)?;
+        Some(buffer_snapshot.language_indent_size_at(offset, cx))
+    }
+
+    pub fn is_dirty(&self) -> bool {
+        self.is_dirty
+    }
+
+    pub fn has_conflict(&self) -> bool {
+        self.has_conflict
+    }
+
+    pub fn diagnostic_group<'a, O>(
+        &'a self,
+        group_id: usize,
+    ) -> impl Iterator<Item = DiagnosticEntry<O>> + 'a
+    where
+        O: text::FromAnchor + 'a,
+    {
+        self.as_singleton()
+            .into_iter()
+            .flat_map(move |(_, _, buffer)| buffer.diagnostic_group(group_id))
+    }
+
+    pub fn diagnostics_in_range<'a, T, O>(
+        &'a self,
+        range: Range<T>,
+        reversed: bool,
+    ) -> impl Iterator<Item = DiagnosticEntry<O>> + 'a
+    where
+        T: 'a + ToOffset,
+        O: 'a + text::FromAnchor + Ord,
+    {
+        self.as_singleton()
+            .into_iter()
+            .flat_map(move |(_, _, buffer)| {
+                buffer.diagnostics_in_range(
+                    range.start.to_offset(self)..range.end.to_offset(self),
+                    reversed,
+                )
+            })
+    }
+
+    pub fn has_git_diffs(&self) -> bool {
+        for excerpt in self.excerpts.iter() {
+            if !excerpt.buffer.git_diff.is_empty() {
+                return true;
+            }
+        }
+        false
+    }
+
+    pub fn git_diff_hunks_in_range_rev<'a>(
+        &'a self,
+        row_range: Range<u32>,
+    ) -> impl 'a + Iterator<Item = DiffHunk<u32>> {
+        let mut cursor = self.excerpts.cursor::<Point>();
+
+        cursor.seek(&Point::new(row_range.end, 0), Bias::Left, &());
+        if cursor.item().is_none() {
+            cursor.prev(&());
+        }
+
+        std::iter::from_fn(move || {
+            let excerpt = cursor.item()?;
+            let multibuffer_start = *cursor.start();
+            let multibuffer_end = multibuffer_start + excerpt.text_summary.lines;
+            if multibuffer_start.row >= row_range.end {
+                return None;
+            }
+
+            let mut buffer_start = excerpt.range.context.start;
+            let mut buffer_end = excerpt.range.context.end;
+            let excerpt_start_point = buffer_start.to_point(&excerpt.buffer);
+            let excerpt_end_point = excerpt_start_point + excerpt.text_summary.lines;
+
+            if row_range.start > multibuffer_start.row {
+                let buffer_start_point =
+                    excerpt_start_point + Point::new(row_range.start - multibuffer_start.row, 0);
+                buffer_start = excerpt.buffer.anchor_before(buffer_start_point);
+            }
+
+            if row_range.end < multibuffer_end.row {
+                let buffer_end_point =
+                    excerpt_start_point + Point::new(row_range.end - multibuffer_start.row, 0);
+                buffer_end = excerpt.buffer.anchor_before(buffer_end_point);
+            }
+
+            let buffer_hunks = excerpt
+                .buffer
+                .git_diff_hunks_intersecting_range_rev(buffer_start..buffer_end)
+                .filter_map(move |hunk| {
+                    let start = multibuffer_start.row
+                        + hunk
+                            .buffer_range
+                            .start
+                            .saturating_sub(excerpt_start_point.row);
+                    let end = multibuffer_start.row
+                        + hunk
+                            .buffer_range
+                            .end
+                            .min(excerpt_end_point.row + 1)
+                            .saturating_sub(excerpt_start_point.row);
+
+                    Some(DiffHunk {
+                        buffer_range: start..end,
+                        diff_base_byte_range: hunk.diff_base_byte_range.clone(),
+                    })
+                });
+
+            cursor.prev(&());
+
+            Some(buffer_hunks)
+        })
+        .flatten()
+    }
+
+    pub fn git_diff_hunks_in_range<'a>(
+        &'a self,
+        row_range: Range<u32>,
+    ) -> impl 'a + Iterator<Item = DiffHunk<u32>> {
+        let mut cursor = self.excerpts.cursor::<Point>();
+
+        cursor.seek(&Point::new(row_range.start, 0), Bias::Right, &());
+
+        std::iter::from_fn(move || {
+            let excerpt = cursor.item()?;
+            let multibuffer_start = *cursor.start();
+            let multibuffer_end = multibuffer_start + excerpt.text_summary.lines;
+            if multibuffer_start.row >= row_range.end {
+                return None;
+            }
+
+            let mut buffer_start = excerpt.range.context.start;
+            let mut buffer_end = excerpt.range.context.end;
+            let excerpt_start_point = buffer_start.to_point(&excerpt.buffer);
+            let excerpt_end_point = excerpt_start_point + excerpt.text_summary.lines;
+
+            if row_range.start > multibuffer_start.row {
+                let buffer_start_point =
+                    excerpt_start_point + Point::new(row_range.start - multibuffer_start.row, 0);
+                buffer_start = excerpt.buffer.anchor_before(buffer_start_point);
+            }
+
+            if row_range.end < multibuffer_end.row {
+                let buffer_end_point =
+                    excerpt_start_point + Point::new(row_range.end - multibuffer_start.row, 0);
+                buffer_end = excerpt.buffer.anchor_before(buffer_end_point);
+            }
+
+            let buffer_hunks = excerpt
+                .buffer
+                .git_diff_hunks_intersecting_range(buffer_start..buffer_end)
+                .filter_map(move |hunk| {
+                    let start = multibuffer_start.row
+                        + hunk
+                            .buffer_range
+                            .start
+                            .saturating_sub(excerpt_start_point.row);
+                    let end = multibuffer_start.row
+                        + hunk
+                            .buffer_range
+                            .end
+                            .min(excerpt_end_point.row + 1)
+                            .saturating_sub(excerpt_start_point.row);
+
+                    Some(DiffHunk {
+                        buffer_range: start..end,
+                        diff_base_byte_range: hunk.diff_base_byte_range.clone(),
+                    })
+                });
+
+            cursor.next(&());
+
+            Some(buffer_hunks)
+        })
+        .flatten()
+    }
+
+    pub fn range_for_syntax_ancestor<T: ToOffset>(&self, range: Range<T>) -> Option<Range<usize>> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+
+        self.excerpt_containing(range.clone())
+            .and_then(|(excerpt, excerpt_offset)| {
+                let excerpt_buffer_start = excerpt.range.context.start.to_offset(&excerpt.buffer);
+                let excerpt_buffer_end = excerpt_buffer_start + excerpt.text_summary.len;
+
+                let start_in_buffer =
+                    excerpt_buffer_start + range.start.saturating_sub(excerpt_offset);
+                let end_in_buffer = excerpt_buffer_start + range.end.saturating_sub(excerpt_offset);
+                let mut ancestor_buffer_range = excerpt
+                    .buffer
+                    .range_for_syntax_ancestor(start_in_buffer..end_in_buffer)?;
+                ancestor_buffer_range.start =
+                    cmp::max(ancestor_buffer_range.start, excerpt_buffer_start);
+                ancestor_buffer_range.end = cmp::min(ancestor_buffer_range.end, excerpt_buffer_end);
+
+                let start = excerpt_offset + (ancestor_buffer_range.start - excerpt_buffer_start);
+                let end = excerpt_offset + (ancestor_buffer_range.end - excerpt_buffer_start);
+                Some(start..end)
+            })
+    }
+
+    pub fn outline(&self, theme: Option<&SyntaxTheme>) -> Option<Outline<Anchor>> {
+        let (excerpt_id, _, buffer) = self.as_singleton()?;
+        let outline = buffer.outline(theme)?;
+        Some(Outline::new(
+            outline
+                .items
+                .into_iter()
+                .map(|item| OutlineItem {
+                    depth: item.depth,
+                    range: self.anchor_in_excerpt(excerpt_id.clone(), item.range.start)
+                        ..self.anchor_in_excerpt(excerpt_id.clone(), item.range.end),
+                    text: item.text,
+                    highlight_ranges: item.highlight_ranges,
+                    name_ranges: item.name_ranges,
+                })
+                .collect(),
+        ))
+    }
+
+    pub fn symbols_containing<T: ToOffset>(
+        &self,
+        offset: T,
+        theme: Option<&SyntaxTheme>,
+    ) -> Option<(u64, Vec<OutlineItem<Anchor>>)> {
+        let anchor = self.anchor_before(offset);
+        let excerpt_id = anchor.excerpt_id;
+        let excerpt = self.excerpt(excerpt_id)?;
+        Some((
+            excerpt.buffer_id,
+            excerpt
+                .buffer
+                .symbols_containing(anchor.text_anchor, theme)
+                .into_iter()
+                .flatten()
+                .map(|item| OutlineItem {
+                    depth: item.depth,
+                    range: self.anchor_in_excerpt(excerpt_id, item.range.start)
+                        ..self.anchor_in_excerpt(excerpt_id, item.range.end),
+                    text: item.text,
+                    highlight_ranges: item.highlight_ranges,
+                    name_ranges: item.name_ranges,
+                })
+                .collect(),
+        ))
+    }
+
+    fn excerpt_locator_for_id<'a>(&'a self, id: ExcerptId) -> &'a Locator {
+        if id == ExcerptId::min() {
+            Locator::min_ref()
+        } else if id == ExcerptId::max() {
+            Locator::max_ref()
+        } else {
+            let mut cursor = self.excerpt_ids.cursor::<ExcerptId>();
+            cursor.seek(&id, Bias::Left, &());
+            if let Some(entry) = cursor.item() {
+                if entry.id == id {
+                    return &entry.locator;
+                }
+            }
+            panic!("invalid excerpt id {:?}", id)
+        }
+    }
+
+    pub fn buffer_id_for_excerpt(&self, excerpt_id: ExcerptId) -> Option<u64> {
+        Some(self.excerpt(excerpt_id)?.buffer_id)
+    }
+
+    pub fn buffer_for_excerpt(&self, excerpt_id: ExcerptId) -> Option<&BufferSnapshot> {
+        Some(&self.excerpt(excerpt_id)?.buffer)
+    }
+
+    fn excerpt<'a>(&'a self, excerpt_id: ExcerptId) -> Option<&'a Excerpt> {
+        let mut cursor = self.excerpts.cursor::<Option<&Locator>>();
+        let locator = self.excerpt_locator_for_id(excerpt_id);
+        cursor.seek(&Some(locator), Bias::Left, &());
+        if let Some(excerpt) = cursor.item() {
+            if excerpt.id == excerpt_id {
+                return Some(excerpt);
+            }
+        }
+        None
+    }
+
+    /// Returns the excerpt containing range and its offset start within the multibuffer or none if `range` spans multiple excerpts
+    fn excerpt_containing<'a, T: ToOffset>(
+        &'a self,
+        range: Range<T>,
+    ) -> Option<(&'a Excerpt, usize)> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+
+        let mut cursor = self.excerpts.cursor::<usize>();
+        cursor.seek(&range.start, Bias::Right, &());
+        let start_excerpt = cursor.item();
+
+        if range.start == range.end {
+            return start_excerpt.map(|excerpt| (excerpt, *cursor.start()));
+        }
+
+        cursor.seek(&range.end, Bias::Right, &());
+        let end_excerpt = cursor.item();
+
+        start_excerpt
+            .zip(end_excerpt)
+            .and_then(|(start_excerpt, end_excerpt)| {
+                if start_excerpt.id != end_excerpt.id {
+                    return None;
+                }
+
+                Some((start_excerpt, *cursor.start()))
+            })
+    }
+
+    pub fn remote_selections_in_range<'a>(
+        &'a self,
+        range: &'a Range<Anchor>,
+    ) -> impl 'a + Iterator<Item = (ReplicaId, bool, CursorShape, Selection<Anchor>)> {
+        let mut cursor = self.excerpts.cursor::<ExcerptSummary>();
+        let start_locator = self.excerpt_locator_for_id(range.start.excerpt_id);
+        let end_locator = self.excerpt_locator_for_id(range.end.excerpt_id);
+        cursor.seek(start_locator, Bias::Left, &());
+        cursor
+            .take_while(move |excerpt| excerpt.locator <= *end_locator)
+            .flat_map(move |excerpt| {
+                let mut query_range = excerpt.range.context.start..excerpt.range.context.end;
+                if excerpt.id == range.start.excerpt_id {
+                    query_range.start = range.start.text_anchor;
+                }
+                if excerpt.id == range.end.excerpt_id {
+                    query_range.end = range.end.text_anchor;
+                }
+
+                excerpt
+                    .buffer
+                    .remote_selections_in_range(query_range)
+                    .flat_map(move |(replica_id, line_mode, cursor_shape, selections)| {
+                        selections.map(move |selection| {
+                            let mut start = Anchor {
+                                buffer_id: Some(excerpt.buffer_id),
+                                excerpt_id: excerpt.id.clone(),
+                                text_anchor: selection.start,
+                            };
+                            let mut end = Anchor {
+                                buffer_id: Some(excerpt.buffer_id),
+                                excerpt_id: excerpt.id.clone(),
+                                text_anchor: selection.end,
+                            };
+                            if range.start.cmp(&start, self).is_gt() {
+                                start = range.start.clone();
+                            }
+                            if range.end.cmp(&end, self).is_lt() {
+                                end = range.end.clone();
+                            }
+
+                            (
+                                replica_id,
+                                line_mode,
+                                cursor_shape,
+                                Selection {
+                                    id: selection.id,
+                                    start,
+                                    end,
+                                    reversed: selection.reversed,
+                                    goal: selection.goal,
+                                },
+                            )
+                        })
+                    })
+            })
+    }
+}
+
+#[cfg(any(test, feature = "test-support"))]
+impl MultiBufferSnapshot {
+    pub fn random_byte_range(&self, start_offset: usize, rng: &mut impl rand::Rng) -> Range<usize> {
+        let end = self.clip_offset(rng.gen_range(start_offset..=self.len()), Bias::Right);
+        let start = self.clip_offset(rng.gen_range(start_offset..=end), Bias::Right);
+        start..end
+    }
+}
+
+impl History {
+    fn start_transaction(&mut self, now: Instant) -> Option<TransactionId> {
+        self.transaction_depth += 1;
+        if self.transaction_depth == 1 {
+            let id = self.next_transaction_id.tick();
+            self.undo_stack.push(Transaction {
+                id,
+                buffer_transactions: Default::default(),
+                first_edit_at: now,
+                last_edit_at: now,
+                suppress_grouping: false,
+            });
+            Some(id)
+        } else {
+            None
+        }
+    }
+
+    fn end_transaction(
+        &mut self,
+        now: Instant,
+        buffer_transactions: HashMap<u64, TransactionId>,
+    ) -> bool {
+        assert_ne!(self.transaction_depth, 0);
+        self.transaction_depth -= 1;
+        if self.transaction_depth == 0 {
+            if buffer_transactions.is_empty() {
+                self.undo_stack.pop();
+                false
+            } else {
+                self.redo_stack.clear();
+                let transaction = self.undo_stack.last_mut().unwrap();
+                transaction.last_edit_at = now;
+                for (buffer_id, transaction_id) in buffer_transactions {
+                    transaction
+                        .buffer_transactions
+                        .entry(buffer_id)
+                        .or_insert(transaction_id);
+                }
+                true
+            }
+        } else {
+            false
+        }
+    }
+
+    fn push_transaction<'a, T>(
+        &mut self,
+        buffer_transactions: T,
+        now: Instant,
+        cx: &mut ModelContext<MultiBuffer>,
+    ) where
+        T: IntoIterator<Item = (&'a Model<Buffer>, &'a language2::Transaction)>,
+    {
+        assert_eq!(self.transaction_depth, 0);
+        let transaction = Transaction {
+            id: self.next_transaction_id.tick(),
+            buffer_transactions: buffer_transactions
+                .into_iter()
+                .map(|(buffer, transaction)| (buffer.read(cx).remote_id(), transaction.id))
+                .collect(),
+            first_edit_at: now,
+            last_edit_at: now,
+            suppress_grouping: false,
+        };
+        if !transaction.buffer_transactions.is_empty() {
+            self.undo_stack.push(transaction);
+            self.redo_stack.clear();
+        }
+    }
+
+    fn finalize_last_transaction(&mut self) {
+        if let Some(transaction) = self.undo_stack.last_mut() {
+            transaction.suppress_grouping = true;
+        }
+    }
+
+    fn forget(&mut self, transaction_id: TransactionId) -> Option<Transaction> {
+        if let Some(ix) = self
+            .undo_stack
+            .iter()
+            .rposition(|transaction| transaction.id == transaction_id)
+        {
+            Some(self.undo_stack.remove(ix))
+        } else if let Some(ix) = self
+            .redo_stack
+            .iter()
+            .rposition(|transaction| transaction.id == transaction_id)
+        {
+            Some(self.redo_stack.remove(ix))
+        } else {
+            None
+        }
+    }
+
+    fn transaction_mut(&mut self, transaction_id: TransactionId) -> Option<&mut Transaction> {
+        self.undo_stack
+            .iter_mut()
+            .find(|transaction| transaction.id == transaction_id)
+            .or_else(|| {
+                self.redo_stack
+                    .iter_mut()
+                    .find(|transaction| transaction.id == transaction_id)
+            })
+    }
+
+    fn pop_undo(&mut self) -> Option<&mut Transaction> {
+        assert_eq!(self.transaction_depth, 0);
+        if let Some(transaction) = self.undo_stack.pop() {
+            self.redo_stack.push(transaction);
+            self.redo_stack.last_mut()
+        } else {
+            None
+        }
+    }
+
+    fn pop_redo(&mut self) -> Option<&mut Transaction> {
+        assert_eq!(self.transaction_depth, 0);
+        if let Some(transaction) = self.redo_stack.pop() {
+            self.undo_stack.push(transaction);
+            self.undo_stack.last_mut()
+        } else {
+            None
+        }
+    }
+
+    fn remove_from_undo(&mut self, transaction_id: TransactionId) -> Option<&Transaction> {
+        let ix = self
+            .undo_stack
+            .iter()
+            .rposition(|transaction| transaction.id == transaction_id)?;
+        let transaction = self.undo_stack.remove(ix);
+        self.redo_stack.push(transaction);
+        self.redo_stack.last()
+    }
+
+    fn group(&mut self) -> Option<TransactionId> {
+        let mut count = 0;
+        let mut transactions = self.undo_stack.iter();
+        if let Some(mut transaction) = transactions.next_back() {
+            while let Some(prev_transaction) = transactions.next_back() {
+                if !prev_transaction.suppress_grouping
+                    && transaction.first_edit_at - prev_transaction.last_edit_at
+                        <= self.group_interval
+                {
+                    transaction = prev_transaction;
+                    count += 1;
+                } else {
+                    break;
+                }
+            }
+        }
+        self.group_trailing(count)
+    }
+
+    fn group_until(&mut self, transaction_id: TransactionId) {
+        let mut count = 0;
+        for transaction in self.undo_stack.iter().rev() {
+            if transaction.id == transaction_id {
+                self.group_trailing(count);
+                break;
+            } else if transaction.suppress_grouping {
+                break;
+            } else {
+                count += 1;
+            }
+        }
+    }
+
+    fn group_trailing(&mut self, n: usize) -> Option<TransactionId> {
+        let new_len = self.undo_stack.len() - n;
+        let (transactions_to_keep, transactions_to_merge) = self.undo_stack.split_at_mut(new_len);
+        if let Some(last_transaction) = transactions_to_keep.last_mut() {
+            if let Some(transaction) = transactions_to_merge.last() {
+                last_transaction.last_edit_at = transaction.last_edit_at;
+            }
+            for to_merge in transactions_to_merge {
+                for (buffer_id, transaction_id) in &to_merge.buffer_transactions {
+                    last_transaction
+                        .buffer_transactions
+                        .entry(*buffer_id)
+                        .or_insert(*transaction_id);
+                }
+            }
+        }
+
+        self.undo_stack.truncate(new_len);
+        self.undo_stack.last().map(|t| t.id)
+    }
+}
+
+impl Excerpt {
+    fn new(
+        id: ExcerptId,
+        locator: Locator,
+        buffer_id: u64,
+        buffer: BufferSnapshot,
+        range: ExcerptRange<text::Anchor>,
+        has_trailing_newline: bool,
+    ) -> Self {
+        Excerpt {
+            id,
+            locator,
+            max_buffer_row: range.context.end.to_point(&buffer).row,
+            text_summary: buffer
+                .text_summary_for_range::<TextSummary, _>(range.context.to_offset(&buffer)),
+            buffer_id,
+            buffer,
+            range,
+            has_trailing_newline,
+        }
+    }
+
+    fn chunks_in_range(&self, range: Range<usize>, language_aware: bool) -> ExcerptChunks {
+        let content_start = self.range.context.start.to_offset(&self.buffer);
+        let chunks_start = content_start + range.start;
+        let chunks_end = content_start + cmp::min(range.end, self.text_summary.len);
+
+        let footer_height = if self.has_trailing_newline
+            && range.start <= self.text_summary.len
+            && range.end > self.text_summary.len
+        {
+            1
+        } else {
+            0
+        };
+
+        let content_chunks = self.buffer.chunks(chunks_start..chunks_end, language_aware);
+
+        ExcerptChunks {
+            content_chunks,
+            footer_height,
+        }
+    }
+
+    fn bytes_in_range(&self, range: Range<usize>) -> ExcerptBytes {
+        let content_start = self.range.context.start.to_offset(&self.buffer);
+        let bytes_start = content_start + range.start;
+        let bytes_end = content_start + cmp::min(range.end, self.text_summary.len);
+        let footer_height = if self.has_trailing_newline
+            && range.start <= self.text_summary.len
+            && range.end > self.text_summary.len
+        {
+            1
+        } else {
+            0
+        };
+        let content_bytes = self.buffer.bytes_in_range(bytes_start..bytes_end);
+
+        ExcerptBytes {
+            content_bytes,
+            footer_height,
+        }
+    }
+
+    fn reversed_bytes_in_range(&self, range: Range<usize>) -> ExcerptBytes {
+        let content_start = self.range.context.start.to_offset(&self.buffer);
+        let bytes_start = content_start + range.start;
+        let bytes_end = content_start + cmp::min(range.end, self.text_summary.len);
+        let footer_height = if self.has_trailing_newline
+            && range.start <= self.text_summary.len
+            && range.end > self.text_summary.len
+        {
+            1
+        } else {
+            0
+        };
+        let content_bytes = self.buffer.reversed_bytes_in_range(bytes_start..bytes_end);
+
+        ExcerptBytes {
+            content_bytes,
+            footer_height,
+        }
+    }
+
+    fn clip_anchor(&self, text_anchor: text::Anchor) -> text::Anchor {
+        if text_anchor
+            .cmp(&self.range.context.start, &self.buffer)
+            .is_lt()
+        {
+            self.range.context.start
+        } else if text_anchor
+            .cmp(&self.range.context.end, &self.buffer)
+            .is_gt()
+        {
+            self.range.context.end
+        } else {
+            text_anchor
+        }
+    }
+
+    fn contains(&self, anchor: &Anchor) -> bool {
+        Some(self.buffer_id) == anchor.buffer_id
+            && self
+                .range
+                .context
+                .start
+                .cmp(&anchor.text_anchor, &self.buffer)
+                .is_le()
+            && self
+                .range
+                .context
+                .end
+                .cmp(&anchor.text_anchor, &self.buffer)
+                .is_ge()
+    }
+}
+
+impl ExcerptId {
+    pub fn min() -> Self {
+        Self(0)
+    }
+
+    pub fn max() -> Self {
+        Self(usize::MAX)
+    }
+
+    pub fn to_proto(&self) -> u64 {
+        self.0 as _
+    }
+
+    pub fn from_proto(proto: u64) -> Self {
+        Self(proto as _)
+    }
+
+    pub fn cmp(&self, other: &Self, snapshot: &MultiBufferSnapshot) -> cmp::Ordering {
+        let a = snapshot.excerpt_locator_for_id(*self);
+        let b = snapshot.excerpt_locator_for_id(*other);
+        a.cmp(&b).then_with(|| self.0.cmp(&other.0))
+    }
+}
+
+impl Into<usize> for ExcerptId {
+    fn into(self) -> usize {
+        self.0
+    }
+}
+
+impl fmt::Debug for Excerpt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Excerpt")
+            .field("id", &self.id)
+            .field("locator", &self.locator)
+            .field("buffer_id", &self.buffer_id)
+            .field("range", &self.range)
+            .field("text_summary", &self.text_summary)
+            .field("has_trailing_newline", &self.has_trailing_newline)
+            .finish()
+    }
+}
+
+impl sum_tree::Item for Excerpt {
+    type Summary = ExcerptSummary;
+
+    fn summary(&self) -> Self::Summary {
+        let mut text = self.text_summary.clone();
+        if self.has_trailing_newline {
+            text += TextSummary::from("\n");
+        }
+        ExcerptSummary {
+            excerpt_id: self.id,
+            excerpt_locator: self.locator.clone(),
+            max_buffer_row: self.max_buffer_row,
+            text,
+        }
+    }
+}
+
+impl sum_tree::Item for ExcerptIdMapping {
+    type Summary = ExcerptId;
+
+    fn summary(&self) -> Self::Summary {
+        self.id
+    }
+}
+
+impl sum_tree::KeyedItem for ExcerptIdMapping {
+    type Key = ExcerptId;
+
+    fn key(&self) -> Self::Key {
+        self.id
+    }
+}
+
+impl sum_tree::Summary for ExcerptId {
+    type Context = ();
+
+    fn add_summary(&mut self, other: &Self, _: &()) {
+        *self = *other;
+    }
+}
+
+impl sum_tree::Summary for ExcerptSummary {
+    type Context = ();
+
+    fn add_summary(&mut self, summary: &Self, _: &()) {
+        debug_assert!(summary.excerpt_locator > self.excerpt_locator);
+        self.excerpt_locator = summary.excerpt_locator.clone();
+        self.text.add_summary(&summary.text, &());
+        self.max_buffer_row = cmp::max(self.max_buffer_row, summary.max_buffer_row);
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for TextSummary {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self += &summary.text;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for usize {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self += summary.text.len;
+    }
+}
+
+impl<'a> sum_tree::SeekTarget<'a, ExcerptSummary, ExcerptSummary> for usize {
+    fn cmp(&self, cursor_location: &ExcerptSummary, _: &()) -> cmp::Ordering {
+        Ord::cmp(self, &cursor_location.text.len)
+    }
+}
+
+impl<'a> sum_tree::SeekTarget<'a, ExcerptSummary, Option<&'a Locator>> for Locator {
+    fn cmp(&self, cursor_location: &Option<&'a Locator>, _: &()) -> cmp::Ordering {
+        Ord::cmp(&Some(self), cursor_location)
+    }
+}
+
+impl<'a> sum_tree::SeekTarget<'a, ExcerptSummary, ExcerptSummary> for Locator {
+    fn cmp(&self, cursor_location: &ExcerptSummary, _: &()) -> cmp::Ordering {
+        Ord::cmp(self, &cursor_location.excerpt_locator)
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for OffsetUtf16 {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self += summary.text.len_utf16;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for Point {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self += summary.text.lines;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for PointUtf16 {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self += summary.text.lines_utf16()
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for Option<&'a Locator> {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self = Some(&summary.excerpt_locator);
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ExcerptSummary> for Option<ExcerptId> {
+    fn add_summary(&mut self, summary: &'a ExcerptSummary, _: &()) {
+        *self = Some(summary.excerpt_id);
+    }
+}
+
+impl<'a> MultiBufferRows<'a> {
+    pub fn seek(&mut self, row: u32) {
+        self.buffer_row_range = 0..0;
+
+        self.excerpts
+            .seek_forward(&Point::new(row, 0), Bias::Right, &());
+        if self.excerpts.item().is_none() {
+            self.excerpts.prev(&());
+
+            if self.excerpts.item().is_none() && row == 0 {
+                self.buffer_row_range = 0..1;
+                return;
+            }
+        }
+
+        if let Some(excerpt) = self.excerpts.item() {
+            let overshoot = row - self.excerpts.start().row;
+            let excerpt_start = excerpt.range.context.start.to_point(&excerpt.buffer).row;
+            self.buffer_row_range.start = excerpt_start + overshoot;
+            self.buffer_row_range.end = excerpt_start + excerpt.text_summary.lines.row + 1;
+        }
+    }
+}
+
+impl<'a> Iterator for MultiBufferRows<'a> {
+    type Item = Option<u32>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if !self.buffer_row_range.is_empty() {
+                let row = Some(self.buffer_row_range.start);
+                self.buffer_row_range.start += 1;
+                return Some(row);
+            }
+            self.excerpts.item()?;
+            self.excerpts.next(&());
+            let excerpt = self.excerpts.item()?;
+            self.buffer_row_range.start = excerpt.range.context.start.to_point(&excerpt.buffer).row;
+            self.buffer_row_range.end =
+                self.buffer_row_range.start + excerpt.text_summary.lines.row + 1;
+        }
+    }
+}
+
+impl<'a> MultiBufferChunks<'a> {
+    pub fn offset(&self) -> usize {
+        self.range.start
+    }
+
+    pub fn seek(&mut self, offset: usize) {
+        self.range.start = offset;
+        self.excerpts.seek(&offset, Bias::Right, &());
+        if let Some(excerpt) = self.excerpts.item() {
+            self.excerpt_chunks = Some(excerpt.chunks_in_range(
+                self.range.start - self.excerpts.start()..self.range.end - self.excerpts.start(),
+                self.language_aware,
+            ));
+        } else {
+            self.excerpt_chunks = None;
+        }
+    }
+}
+
+impl<'a> Iterator for MultiBufferChunks<'a> {
+    type Item = Chunk<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.range.is_empty() {
+            None
+        } else if let Some(chunk) = self.excerpt_chunks.as_mut()?.next() {
+            self.range.start += chunk.text.len();
+            Some(chunk)
+        } else {
+            self.excerpts.next(&());
+            let excerpt = self.excerpts.item()?;
+            self.excerpt_chunks = Some(excerpt.chunks_in_range(
+                0..self.range.end - self.excerpts.start(),
+                self.language_aware,
+            ));
+            self.next()
+        }
+    }
+}
+
+impl<'a> MultiBufferBytes<'a> {
+    fn consume(&mut self, len: usize) {
+        self.range.start += len;
+        self.chunk = &self.chunk[len..];
+
+        if !self.range.is_empty() && self.chunk.is_empty() {
+            if let Some(chunk) = self.excerpt_bytes.as_mut().and_then(|bytes| bytes.next()) {
+                self.chunk = chunk;
+            } else {
+                self.excerpts.next(&());
+                if let Some(excerpt) = self.excerpts.item() {
+                    let mut excerpt_bytes =
+                        excerpt.bytes_in_range(0..self.range.end - self.excerpts.start());
+                    self.chunk = excerpt_bytes.next().unwrap();
+                    self.excerpt_bytes = Some(excerpt_bytes);
+                }
+            }
+        }
+    }
+}
+
+impl<'a> Iterator for MultiBufferBytes<'a> {
+    type Item = &'a [u8];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let chunk = self.chunk;
+        if chunk.is_empty() {
+            None
+        } else {
+            self.consume(chunk.len());
+            Some(chunk)
+        }
+    }
+}
+
+impl<'a> io::Read for MultiBufferBytes<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let len = cmp::min(buf.len(), self.chunk.len());
+        buf[..len].copy_from_slice(&self.chunk[..len]);
+        if len > 0 {
+            self.consume(len);
+        }
+        Ok(len)
+    }
+}
+
+impl<'a> ReversedMultiBufferBytes<'a> {
+    fn consume(&mut self, len: usize) {
+        self.range.end -= len;
+        self.chunk = &self.chunk[..self.chunk.len() - len];
+
+        if !self.range.is_empty() && self.chunk.is_empty() {
+            if let Some(chunk) = self.excerpt_bytes.as_mut().and_then(|bytes| bytes.next()) {
+                self.chunk = chunk;
+            } else {
+                self.excerpts.next(&());
+                if let Some(excerpt) = self.excerpts.item() {
+                    let mut excerpt_bytes =
+                        excerpt.bytes_in_range(0..self.range.end - self.excerpts.start());
+                    self.chunk = excerpt_bytes.next().unwrap();
+                    self.excerpt_bytes = Some(excerpt_bytes);
+                }
+            }
+        }
+    }
+}
+
+impl<'a> io::Read for ReversedMultiBufferBytes<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let len = cmp::min(buf.len(), self.chunk.len());
+        buf[..len].copy_from_slice(&self.chunk[..len]);
+        buf[..len].reverse();
+        if len > 0 {
+            self.consume(len);
+        }
+        Ok(len)
+    }
+}
+impl<'a> Iterator for ExcerptBytes<'a> {
+    type Item = &'a [u8];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(chunk) = self.content_bytes.next() {
+            if !chunk.is_empty() {
+                return Some(chunk);
+            }
+        }
+
+        if self.footer_height > 0 {
+            let result = &NEWLINES[..self.footer_height];
+            self.footer_height = 0;
+            return Some(result);
+        }
+
+        None
+    }
+}
+
+impl<'a> Iterator for ExcerptChunks<'a> {
+    type Item = Chunk<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(chunk) = self.content_chunks.next() {
+            return Some(chunk);
+        }
+
+        if self.footer_height > 0 {
+            let text = unsafe { str::from_utf8_unchecked(&NEWLINES[..self.footer_height]) };
+            self.footer_height = 0;
+            return Some(Chunk {
+                text,
+                ..Default::default()
+            });
+        }
+
+        None
+    }
+}
+
+impl ToOffset for Point {
+    fn to_offset<'a>(&self, snapshot: &MultiBufferSnapshot) -> usize {
+        snapshot.point_to_offset(*self)
+    }
+}
+
+impl ToOffset for usize {
+    fn to_offset<'a>(&self, snapshot: &MultiBufferSnapshot) -> usize {
+        assert!(*self <= snapshot.len(), "offset is out of range");
+        *self
+    }
+}
+
+impl ToOffset for OffsetUtf16 {
+    fn to_offset<'a>(&self, snapshot: &MultiBufferSnapshot) -> usize {
+        snapshot.offset_utf16_to_offset(*self)
+    }
+}
+
+impl ToOffset for PointUtf16 {
+    fn to_offset<'a>(&self, snapshot: &MultiBufferSnapshot) -> usize {
+        snapshot.point_utf16_to_offset(*self)
+    }
+}
+
+impl ToOffsetUtf16 for OffsetUtf16 {
+    fn to_offset_utf16(&self, _snapshot: &MultiBufferSnapshot) -> OffsetUtf16 {
+        *self
+    }
+}
+
+impl ToOffsetUtf16 for usize {
+    fn to_offset_utf16(&self, snapshot: &MultiBufferSnapshot) -> OffsetUtf16 {
+        snapshot.offset_to_offset_utf16(*self)
+    }
+}
+
+impl ToPoint for usize {
+    fn to_point<'a>(&self, snapshot: &MultiBufferSnapshot) -> Point {
+        snapshot.offset_to_point(*self)
+    }
+}
+
+impl ToPoint for Point {
+    fn to_point<'a>(&self, _: &MultiBufferSnapshot) -> Point {
+        *self
+    }
+}
+
+impl ToPointUtf16 for usize {
+    fn to_point_utf16<'a>(&self, snapshot: &MultiBufferSnapshot) -> PointUtf16 {
+        snapshot.offset_to_point_utf16(*self)
+    }
+}
+
+impl ToPointUtf16 for Point {
+    fn to_point_utf16<'a>(&self, snapshot: &MultiBufferSnapshot) -> PointUtf16 {
+        snapshot.point_to_point_utf16(*self)
+    }
+}
+
+impl ToPointUtf16 for PointUtf16 {
+    fn to_point_utf16<'a>(&self, _: &MultiBufferSnapshot) -> PointUtf16 {
+        *self
+    }
+}
+
+fn build_excerpt_ranges<T>(
+    buffer: &BufferSnapshot,
+    ranges: &[Range<T>],
+    context_line_count: u32,
+) -> (Vec<ExcerptRange<Point>>, Vec<usize>)
+where
+    T: text::ToPoint,
+{
+    let max_point = buffer.max_point();
+    let mut range_counts = Vec::new();
+    let mut excerpt_ranges = Vec::new();
+    let mut range_iter = ranges
+        .iter()
+        .map(|range| range.start.to_point(buffer)..range.end.to_point(buffer))
+        .peekable();
+    while let Some(range) = range_iter.next() {
+        let excerpt_start = Point::new(range.start.row.saturating_sub(context_line_count), 0);
+        let mut excerpt_end = Point::new(range.end.row + 1 + context_line_count, 0).min(max_point);
+        let mut ranges_in_excerpt = 1;
+
+        while let Some(next_range) = range_iter.peek() {
+            if next_range.start.row <= excerpt_end.row + context_line_count {
+                excerpt_end =
+                    Point::new(next_range.end.row + 1 + context_line_count, 0).min(max_point);
+                ranges_in_excerpt += 1;
+                range_iter.next();
+            } else {
+                break;
+            }
+        }
+
+        excerpt_ranges.push(ExcerptRange {
+            context: excerpt_start..excerpt_end,
+            primary: Some(range),
+        });
+        range_counts.push(ranges_in_excerpt);
+    }
+
+    (excerpt_ranges, range_counts)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use futures::StreamExt;
+    use gpui2::{AppContext, Context, TestAppContext};
+    use language2::{Buffer, Rope};
+    use parking_lot::RwLock;
+    use rand::prelude::*;
+    use settings2::SettingsStore;
+    use std::env;
+    use util::test::sample_text;
+
+    #[gpui2::test]
+    fn test_singleton(cx: &mut AppContext) {
+        let buffer =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(6, 6, 'a')));
+        let multibuffer = cx.build_model(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot.text(), buffer.read(cx).text());
+
+        assert_eq!(
+            snapshot.buffer_rows(0).collect::<Vec<_>>(),
+            (0..buffer.read(cx).row_count())
+                .map(Some)
+                .collect::<Vec<_>>()
+        );
+
+        buffer.update(cx, |buffer, cx| buffer.edit([(1..3, "XXX\n")], None, cx));
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+
+        assert_eq!(snapshot.text(), buffer.read(cx).text());
+        assert_eq!(
+            snapshot.buffer_rows(0).collect::<Vec<_>>(),
+            (0..buffer.read(cx).row_count())
+                .map(Some)
+                .collect::<Vec<_>>()
+        );
+    }
+
+    #[gpui2::test]
+    fn test_remote(cx: &mut AppContext) {
+        let host_buffer = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "a"));
+        let guest_buffer = cx.build_model(|cx| {
+            let state = host_buffer.read(cx).to_proto();
+            let ops = cx
+                .executor()
+                .block(host_buffer.read(cx).serialize_ops(None, cx));
+            let mut buffer = Buffer::from_proto(1, state, None).unwrap();
+            buffer
+                .apply_ops(
+                    ops.into_iter()
+                        .map(|op| language2::proto::deserialize_operation(op).unwrap()),
+                    cx,
+                )
+                .unwrap();
+            buffer
+        });
+        let multibuffer = cx.build_model(|cx| MultiBuffer::singleton(guest_buffer.clone(), cx));
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot.text(), "a");
+
+        guest_buffer.update(cx, |buffer, cx| buffer.edit([(1..1, "b")], None, cx));
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot.text(), "ab");
+
+        guest_buffer.update(cx, |buffer, cx| buffer.edit([(2..2, "c")], None, cx));
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot.text(), "abc");
+    }
+
+    #[gpui2::test]
+    fn test_excerpt_boundaries_and_clipping(cx: &mut AppContext) {
+        let buffer_1 =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(6, 6, 'a')));
+        let buffer_2 =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(6, 6, 'g')));
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+
+        let events = Arc::new(RwLock::new(Vec::<Event>::new()));
+        multibuffer.update(cx, |_, cx| {
+            let events = events.clone();
+            cx.subscribe(&multibuffer, move |_, _, event, _| {
+                if let Event::Edited { .. } = event {
+                    events.write().push(event.clone())
+                }
+            })
+            .detach();
+        });
+
+        let subscription = multibuffer.update(cx, |multibuffer, cx| {
+            let subscription = multibuffer.subscribe();
+            multibuffer.push_excerpts(
+                buffer_1.clone(),
+                [ExcerptRange {
+                    context: Point::new(1, 2)..Point::new(2, 5),
+                    primary: None,
+                }],
+                cx,
+            );
+            assert_eq!(
+                subscription.consume().into_inner(),
+                [Edit {
+                    old: 0..0,
+                    new: 0..10
+                }]
+            );
+
+            multibuffer.push_excerpts(
+                buffer_1.clone(),
+                [ExcerptRange {
+                    context: Point::new(3, 3)..Point::new(4, 4),
+                    primary: None,
+                }],
+                cx,
+            );
+            multibuffer.push_excerpts(
+                buffer_2.clone(),
+                [ExcerptRange {
+                    context: Point::new(3, 1)..Point::new(3, 3),
+                    primary: None,
+                }],
+                cx,
+            );
+            assert_eq!(
+                subscription.consume().into_inner(),
+                [Edit {
+                    old: 10..10,
+                    new: 10..22
+                }]
+            );
+
+            subscription
+        });
+
+        // Adding excerpts emits an edited event.
+        assert_eq!(
+            events.read().as_slice(),
+            &[
+                Event::Edited {
+                    sigleton_buffer_edited: false
+                },
+                Event::Edited {
+                    sigleton_buffer_edited: false
+                },
+                Event::Edited {
+                    sigleton_buffer_edited: false
+                }
+            ]
+        );
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(
+            snapshot.text(),
+            concat!(
+                "bbbb\n",  // Preserve newlines
+                "ccccc\n", //
+                "ddd\n",   //
+                "eeee\n",  //
+                "jj"       //
+            )
+        );
+        assert_eq!(
+            snapshot.buffer_rows(0).collect::<Vec<_>>(),
+            [Some(1), Some(2), Some(3), Some(4), Some(3)]
+        );
+        assert_eq!(
+            snapshot.buffer_rows(2).collect::<Vec<_>>(),
+            [Some(3), Some(4), Some(3)]
+        );
+        assert_eq!(snapshot.buffer_rows(4).collect::<Vec<_>>(), [Some(3)]);
+        assert_eq!(snapshot.buffer_rows(5).collect::<Vec<_>>(), []);
+
+        assert_eq!(
+            boundaries_in_range(Point::new(0, 0)..Point::new(4, 2), &snapshot),
+            &[
+                (0, "bbbb\nccccc".to_string(), true),
+                (2, "ddd\neeee".to_string(), false),
+                (4, "jj".to_string(), true),
+            ]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(0, 0)..Point::new(2, 0), &snapshot),
+            &[(0, "bbbb\nccccc".to_string(), true)]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(1, 0)..Point::new(1, 5), &snapshot),
+            &[]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(1, 0)..Point::new(2, 0), &snapshot),
+            &[]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(1, 0)..Point::new(4, 0), &snapshot),
+            &[(2, "ddd\neeee".to_string(), false)]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(1, 0)..Point::new(4, 0), &snapshot),
+            &[(2, "ddd\neeee".to_string(), false)]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(2, 0)..Point::new(3, 0), &snapshot),
+            &[(2, "ddd\neeee".to_string(), false)]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(4, 0)..Point::new(4, 2), &snapshot),
+            &[(4, "jj".to_string(), true)]
+        );
+        assert_eq!(
+            boundaries_in_range(Point::new(4, 2)..Point::new(4, 2), &snapshot),
+            &[]
+        );
+
+        buffer_1.update(cx, |buffer, cx| {
+            let text = "\n";
+            buffer.edit(
+                [
+                    (Point::new(0, 0)..Point::new(0, 0), text),
+                    (Point::new(2, 1)..Point::new(2, 3), text),
+                ],
+                None,
+                cx,
+            );
+        });
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(
+            snapshot.text(),
+            concat!(
+                "bbbb\n", // Preserve newlines
+                "c\n",    //
+                "cc\n",   //
+                "ddd\n",  //
+                "eeee\n", //
+                "jj"      //
+            )
+        );
+
+        assert_eq!(
+            subscription.consume().into_inner(),
+            [Edit {
+                old: 6..8,
+                new: 6..7
+            }]
+        );
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(
+            snapshot.clip_point(Point::new(0, 5), Bias::Left),
+            Point::new(0, 4)
+        );
+        assert_eq!(
+            snapshot.clip_point(Point::new(0, 5), Bias::Right),
+            Point::new(0, 4)
+        );
+        assert_eq!(
+            snapshot.clip_point(Point::new(5, 1), Bias::Right),
+            Point::new(5, 1)
+        );
+        assert_eq!(
+            snapshot.clip_point(Point::new(5, 2), Bias::Right),
+            Point::new(5, 2)
+        );
+        assert_eq!(
+            snapshot.clip_point(Point::new(5, 3), Bias::Right),
+            Point::new(5, 2)
+        );
+
+        let snapshot = multibuffer.update(cx, |multibuffer, cx| {
+            let (buffer_2_excerpt_id, _) =
+                multibuffer.excerpts_for_buffer(&buffer_2, cx)[0].clone();
+            multibuffer.remove_excerpts([buffer_2_excerpt_id], cx);
+            multibuffer.snapshot(cx)
+        });
+
+        assert_eq!(
+            snapshot.text(),
+            concat!(
+                "bbbb\n", // Preserve newlines
+                "c\n",    //
+                "cc\n",   //
+                "ddd\n",  //
+                "eeee",   //
+            )
+        );
+
+        fn boundaries_in_range(
+            range: Range<Point>,
+            snapshot: &MultiBufferSnapshot,
+        ) -> Vec<(u32, String, bool)> {
+            snapshot
+                .excerpt_boundaries_in_range(range)
+                .map(|boundary| {
+                    (
+                        boundary.row,
+                        boundary
+                            .buffer
+                            .text_for_range(boundary.range.context)
+                            .collect::<String>(),
+                        boundary.starts_new_buffer,
+                    )
+                })
+                .collect::<Vec<_>>()
+        }
+    }
+
+    #[gpui2::test]
+    fn test_excerpt_events(cx: &mut AppContext) {
+        let buffer_1 =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(10, 3, 'a')));
+        let buffer_2 =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(10, 3, 'm')));
+
+        let leader_multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+        let follower_multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+        let follower_edit_event_count = Arc::new(RwLock::new(0));
+
+        follower_multibuffer.update(cx, |_, cx| {
+            let follower_edit_event_count = follower_edit_event_count.clone();
+            cx.subscribe(
+                &leader_multibuffer,
+                move |follower, _, event, cx| match event.clone() {
+                    Event::ExcerptsAdded {
+                        buffer,
+                        predecessor,
+                        excerpts,
+                    } => follower.insert_excerpts_with_ids_after(predecessor, buffer, excerpts, cx),
+                    Event::ExcerptsRemoved { ids } => follower.remove_excerpts(ids, cx),
+                    Event::Edited { .. } => {
+                        *follower_edit_event_count.write() += 1;
+                    }
+                    _ => {}
+                },
+            )
+            .detach();
+        });
+
+        leader_multibuffer.update(cx, |leader, cx| {
+            leader.push_excerpts(
+                buffer_1.clone(),
+                [
+                    ExcerptRange {
+                        context: 0..8,
+                        primary: None,
+                    },
+                    ExcerptRange {
+                        context: 12..16,
+                        primary: None,
+                    },
+                ],
+                cx,
+            );
+            leader.insert_excerpts_after(
+                leader.excerpt_ids()[0],
+                buffer_2.clone(),
+                [
+                    ExcerptRange {
+                        context: 0..5,
+                        primary: None,
+                    },
+                    ExcerptRange {
+                        context: 10..15,
+                        primary: None,
+                    },
+                ],
+                cx,
+            )
+        });
+        assert_eq!(
+            leader_multibuffer.read(cx).snapshot(cx).text(),
+            follower_multibuffer.read(cx).snapshot(cx).text(),
+        );
+        assert_eq!(*follower_edit_event_count.read(), 2);
+
+        leader_multibuffer.update(cx, |leader, cx| {
+            let excerpt_ids = leader.excerpt_ids();
+            leader.remove_excerpts([excerpt_ids[1], excerpt_ids[3]], cx);
+        });
+        assert_eq!(
+            leader_multibuffer.read(cx).snapshot(cx).text(),
+            follower_multibuffer.read(cx).snapshot(cx).text(),
+        );
+        assert_eq!(*follower_edit_event_count.read(), 3);
+
+        // Removing an empty set of excerpts is a noop.
+        leader_multibuffer.update(cx, |leader, cx| {
+            leader.remove_excerpts([], cx);
+        });
+        assert_eq!(
+            leader_multibuffer.read(cx).snapshot(cx).text(),
+            follower_multibuffer.read(cx).snapshot(cx).text(),
+        );
+        assert_eq!(*follower_edit_event_count.read(), 3);
+
+        // Adding an empty set of excerpts is a noop.
+        leader_multibuffer.update(cx, |leader, cx| {
+            leader.push_excerpts::<usize>(buffer_2.clone(), [], cx);
+        });
+        assert_eq!(
+            leader_multibuffer.read(cx).snapshot(cx).text(),
+            follower_multibuffer.read(cx).snapshot(cx).text(),
+        );
+        assert_eq!(*follower_edit_event_count.read(), 3);
+
+        leader_multibuffer.update(cx, |leader, cx| {
+            leader.clear(cx);
+        });
+        assert_eq!(
+            leader_multibuffer.read(cx).snapshot(cx).text(),
+            follower_multibuffer.read(cx).snapshot(cx).text(),
+        );
+        assert_eq!(*follower_edit_event_count.read(), 4);
+    }
+
+    #[gpui2::test]
+    fn test_push_excerpts_with_context_lines(cx: &mut AppContext) {
+        let buffer =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(20, 3, 'a')));
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+        let anchor_ranges = multibuffer.update(cx, |multibuffer, cx| {
+            multibuffer.push_excerpts_with_context_lines(
+                buffer.clone(),
+                vec![
+                    Point::new(3, 2)..Point::new(4, 2),
+                    Point::new(7, 1)..Point::new(7, 3),
+                    Point::new(15, 0)..Point::new(15, 0),
+                ],
+                2,
+                cx,
+            )
+        });
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(
+            snapshot.text(),
+            "bbb\nccc\nddd\neee\nfff\nggg\nhhh\niii\njjj\n\nnnn\nooo\nppp\nqqq\nrrr\n"
+        );
+
+        assert_eq!(
+            anchor_ranges
+                .iter()
+                .map(|range| range.to_point(&snapshot))
+                .collect::<Vec<_>>(),
+            vec![
+                Point::new(2, 2)..Point::new(3, 2),
+                Point::new(6, 1)..Point::new(6, 3),
+                Point::new(12, 0)..Point::new(12, 0)
+            ]
+        );
+    }
+
+    #[gpui2::test]
+    async fn test_stream_excerpts_with_context_lines(cx: &mut TestAppContext) {
+        let buffer =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), sample_text(20, 3, 'a')));
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+        let anchor_ranges = multibuffer.update(cx, |multibuffer, cx| {
+            let snapshot = buffer.read(cx);
+            let ranges = vec![
+                snapshot.anchor_before(Point::new(3, 2))..snapshot.anchor_before(Point::new(4, 2)),
+                snapshot.anchor_before(Point::new(7, 1))..snapshot.anchor_before(Point::new(7, 3)),
+                snapshot.anchor_before(Point::new(15, 0))
+                    ..snapshot.anchor_before(Point::new(15, 0)),
+            ];
+            multibuffer.stream_excerpts_with_context_lines(buffer.clone(), ranges, 2, cx)
+        });
+
+        let anchor_ranges = anchor_ranges.collect::<Vec<_>>().await;
+
+        let snapshot = multibuffer.update(cx, |multibuffer, cx| multibuffer.snapshot(cx));
+        assert_eq!(
+            snapshot.text(),
+            "bbb\nccc\nddd\neee\nfff\nggg\nhhh\niii\njjj\n\nnnn\nooo\nppp\nqqq\nrrr\n"
+        );
+
+        assert_eq!(
+            anchor_ranges
+                .iter()
+                .map(|range| range.to_point(&snapshot))
+                .collect::<Vec<_>>(),
+            vec![
+                Point::new(2, 2)..Point::new(3, 2),
+                Point::new(6, 1)..Point::new(6, 3),
+                Point::new(12, 0)..Point::new(12, 0)
+            ]
+        );
+    }
+
+    #[gpui2::test]
+    fn test_empty_multibuffer(cx: &mut AppContext) {
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot.text(), "");
+        assert_eq!(snapshot.buffer_rows(0).collect::<Vec<_>>(), &[Some(0)]);
+        assert_eq!(snapshot.buffer_rows(1).collect::<Vec<_>>(), &[]);
+    }
+
+    #[gpui2::test]
+    fn test_singleton_multibuffer_anchors(cx: &mut AppContext) {
+        let buffer = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "abcd"));
+        let multibuffer = cx.build_model(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+        let old_snapshot = multibuffer.read(cx).snapshot(cx);
+        buffer.update(cx, |buffer, cx| {
+            buffer.edit([(0..0, "X")], None, cx);
+            buffer.edit([(5..5, "Y")], None, cx);
+        });
+        let new_snapshot = multibuffer.read(cx).snapshot(cx);
+
+        assert_eq!(old_snapshot.text(), "abcd");
+        assert_eq!(new_snapshot.text(), "XabcdY");
+
+        assert_eq!(old_snapshot.anchor_before(0).to_offset(&new_snapshot), 0);
+        assert_eq!(old_snapshot.anchor_after(0).to_offset(&new_snapshot), 1);
+        assert_eq!(old_snapshot.anchor_before(4).to_offset(&new_snapshot), 5);
+        assert_eq!(old_snapshot.anchor_after(4).to_offset(&new_snapshot), 6);
+    }
+
+    #[gpui2::test]
+    fn test_multibuffer_anchors(cx: &mut AppContext) {
+        let buffer_1 = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "abcd"));
+        let buffer_2 = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "efghi"));
+        let multibuffer = cx.build_model(|cx| {
+            let mut multibuffer = MultiBuffer::new(0);
+            multibuffer.push_excerpts(
+                buffer_1.clone(),
+                [ExcerptRange {
+                    context: 0..4,
+                    primary: None,
+                }],
+                cx,
+            );
+            multibuffer.push_excerpts(
+                buffer_2.clone(),
+                [ExcerptRange {
+                    context: 0..5,
+                    primary: None,
+                }],
+                cx,
+            );
+            multibuffer
+        });
+        let old_snapshot = multibuffer.read(cx).snapshot(cx);
+
+        assert_eq!(old_snapshot.anchor_before(0).to_offset(&old_snapshot), 0);
+        assert_eq!(old_snapshot.anchor_after(0).to_offset(&old_snapshot), 0);
+        assert_eq!(Anchor::min().to_offset(&old_snapshot), 0);
+        assert_eq!(Anchor::min().to_offset(&old_snapshot), 0);
+        assert_eq!(Anchor::max().to_offset(&old_snapshot), 10);
+        assert_eq!(Anchor::max().to_offset(&old_snapshot), 10);
+
+        buffer_1.update(cx, |buffer, cx| {
+            buffer.edit([(0..0, "W")], None, cx);
+            buffer.edit([(5..5, "X")], None, cx);
+        });
+        buffer_2.update(cx, |buffer, cx| {
+            buffer.edit([(0..0, "Y")], None, cx);
+            buffer.edit([(6..6, "Z")], None, cx);
+        });
+        let new_snapshot = multibuffer.read(cx).snapshot(cx);
+
+        assert_eq!(old_snapshot.text(), "abcd\nefghi");
+        assert_eq!(new_snapshot.text(), "WabcdX\nYefghiZ");
+
+        assert_eq!(old_snapshot.anchor_before(0).to_offset(&new_snapshot), 0);
+        assert_eq!(old_snapshot.anchor_after(0).to_offset(&new_snapshot), 1);
+        assert_eq!(old_snapshot.anchor_before(1).to_offset(&new_snapshot), 2);
+        assert_eq!(old_snapshot.anchor_after(1).to_offset(&new_snapshot), 2);
+        assert_eq!(old_snapshot.anchor_before(2).to_offset(&new_snapshot), 3);
+        assert_eq!(old_snapshot.anchor_after(2).to_offset(&new_snapshot), 3);
+        assert_eq!(old_snapshot.anchor_before(5).to_offset(&new_snapshot), 7);
+        assert_eq!(old_snapshot.anchor_after(5).to_offset(&new_snapshot), 8);
+        assert_eq!(old_snapshot.anchor_before(10).to_offset(&new_snapshot), 13);
+        assert_eq!(old_snapshot.anchor_after(10).to_offset(&new_snapshot), 14);
+    }
+
+    #[gpui2::test]
+    fn test_resolving_anchors_after_replacing_their_excerpts(cx: &mut AppContext) {
+        let buffer_1 = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "abcd"));
+        let buffer_2 =
+            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "ABCDEFGHIJKLMNOP"));
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+
+        // Create an insertion id in buffer 1 that doesn't exist in buffer 2.
+        // Add an excerpt from buffer 1 that spans this new insertion.
+        buffer_1.update(cx, |buffer, cx| buffer.edit([(4..4, "123")], None, cx));
+        let excerpt_id_1 = multibuffer.update(cx, |multibuffer, cx| {
+            multibuffer
+                .push_excerpts(
+                    buffer_1.clone(),
+                    [ExcerptRange {
+                        context: 0..7,
+                        primary: None,
+                    }],
+                    cx,
+                )
+                .pop()
+                .unwrap()
+        });
+
+        let snapshot_1 = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot_1.text(), "abcd123");
+
+        // Replace the buffer 1 excerpt with new excerpts from buffer 2.
+        let (excerpt_id_2, excerpt_id_3) = multibuffer.update(cx, |multibuffer, cx| {
+            multibuffer.remove_excerpts([excerpt_id_1], cx);
+            let mut ids = multibuffer
+                .push_excerpts(
+                    buffer_2.clone(),
+                    [
+                        ExcerptRange {
+                            context: 0..4,
+                            primary: None,
+                        },
+                        ExcerptRange {
+                            context: 6..10,
+                            primary: None,
+                        },
+                        ExcerptRange {
+                            context: 12..16,
+                            primary: None,
+                        },
+                    ],
+                    cx,
+                )
+                .into_iter();
+            (ids.next().unwrap(), ids.next().unwrap())
+        });
+        let snapshot_2 = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot_2.text(), "ABCD\nGHIJ\nMNOP");
+
+        // The old excerpt id doesn't get reused.
+        assert_ne!(excerpt_id_2, excerpt_id_1);
+
+        // Resolve some anchors from the previous snapshot in the new snapshot.
+        // The current excerpts are from a different buffer, so we don't attempt to
+        // resolve the old text anchor in the new buffer.
+        assert_eq!(
+            snapshot_2.summary_for_anchor::<usize>(&snapshot_1.anchor_before(2)),
+            0
+        );
+        assert_eq!(
+            snapshot_2.summaries_for_anchors::<usize, _>(&[
+                snapshot_1.anchor_before(2),
+                snapshot_1.anchor_after(3)
+            ]),
+            vec![0, 0]
+        );
+
+        // Refresh anchors from the old snapshot. The return value indicates that both
+        // anchors lost their original excerpt.
+        let refresh =
+            snapshot_2.refresh_anchors(&[snapshot_1.anchor_before(2), snapshot_1.anchor_after(3)]);
+        assert_eq!(
+            refresh,
+            &[
+                (0, snapshot_2.anchor_before(0), false),
+                (1, snapshot_2.anchor_after(0), false),
+            ]
+        );
+
+        // Replace the middle excerpt with a smaller excerpt in buffer 2,
+        // that intersects the old excerpt.
+        let excerpt_id_5 = multibuffer.update(cx, |multibuffer, cx| {
+            multibuffer.remove_excerpts([excerpt_id_3], cx);
+            multibuffer
+                .insert_excerpts_after(
+                    excerpt_id_2,
+                    buffer_2.clone(),
+                    [ExcerptRange {
+                        context: 5..8,
+                        primary: None,
+                    }],
+                    cx,
+                )
+                .pop()
+                .unwrap()
+        });
+
+        let snapshot_3 = multibuffer.read(cx).snapshot(cx);
+        assert_eq!(snapshot_3.text(), "ABCD\nFGH\nMNOP");
+        assert_ne!(excerpt_id_5, excerpt_id_3);
+
+        // Resolve some anchors from the previous snapshot in the new snapshot.
+        // The third anchor can't be resolved, since its excerpt has been removed,
+        // so it resolves to the same position as its predecessor.
+        let anchors = [
+            snapshot_2.anchor_before(0),
+            snapshot_2.anchor_after(2),
+            snapshot_2.anchor_after(6),
+            snapshot_2.anchor_after(14),
+        ];
+        assert_eq!(
+            snapshot_3.summaries_for_anchors::<usize, _>(&anchors),
+            &[0, 2, 9, 13]
+        );
+
+        let new_anchors = snapshot_3.refresh_anchors(&anchors);
+        assert_eq!(
+            new_anchors.iter().map(|a| (a.0, a.2)).collect::<Vec<_>>(),
+            &[(0, true), (1, true), (2, true), (3, true)]
+        );
+        assert_eq!(
+            snapshot_3.summaries_for_anchors::<usize, _>(new_anchors.iter().map(|a| &a.1)),
+            &[0, 2, 7, 13]
+        );
+    }
+
+    #[gpui2::test(iterations = 100)]
+    fn test_random_multibuffer(cx: &mut AppContext, mut rng: StdRng) {
+        let operations = env::var("OPERATIONS")
+            .map(|i| i.parse().expect("invalid `OPERATIONS` variable"))
+            .unwrap_or(10);
+
+        let mut buffers: Vec<Model<Buffer>> = Vec::new();
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+        let mut excerpt_ids = Vec::<ExcerptId>::new();
+        let mut expected_excerpts = Vec::<(Model<Buffer>, Range<text::Anchor>)>::new();
+        let mut anchors = Vec::new();
+        let mut old_versions = Vec::new();
+
+        for _ in 0..operations {
+            match rng.gen_range(0..100) {
+                0..=19 if !buffers.is_empty() => {
+                    let buffer = buffers.choose(&mut rng).unwrap();
+                    buffer.update(cx, |buf, cx| buf.randomly_edit(&mut rng, 5, cx));
+                }
+                20..=29 if !expected_excerpts.is_empty() => {
+                    let mut ids_to_remove = vec![];
+                    for _ in 0..rng.gen_range(1..=3) {
+                        if expected_excerpts.is_empty() {
+                            break;
+                        }
+
+                        let ix = rng.gen_range(0..expected_excerpts.len());
+                        ids_to_remove.push(excerpt_ids.remove(ix));
+                        let (buffer, range) = expected_excerpts.remove(ix);
+                        let buffer = buffer.read(cx);
+                        log::info!(
+                            "Removing excerpt {}: {:?}",
+                            ix,
+                            buffer
+                                .text_for_range(range.to_offset(buffer))
+                                .collect::<String>(),
+                        );
+                    }
+                    let snapshot = multibuffer.read(cx).read(cx);
+                    ids_to_remove.sort_unstable_by(|a, b| a.cmp(&b, &snapshot));
+                    drop(snapshot);
+                    multibuffer.update(cx, |multibuffer, cx| {
+                        multibuffer.remove_excerpts(ids_to_remove, cx)
+                    });
+                }
+                30..=39 if !expected_excerpts.is_empty() => {
+                    let multibuffer = multibuffer.read(cx).read(cx);
+                    let offset =
+                        multibuffer.clip_offset(rng.gen_range(0..=multibuffer.len()), Bias::Left);
+                    let bias = if rng.gen() { Bias::Left } else { Bias::Right };
+                    log::info!("Creating anchor at {} with bias {:?}", offset, bias);
+                    anchors.push(multibuffer.anchor_at(offset, bias));
+                    anchors.sort_by(|a, b| a.cmp(b, &multibuffer));
+                }
+                40..=44 if !anchors.is_empty() => {
+                    let multibuffer = multibuffer.read(cx).read(cx);
+                    let prev_len = anchors.len();
+                    anchors = multibuffer
+                        .refresh_anchors(&anchors)
+                        .into_iter()
+                        .map(|a| a.1)
+                        .collect();
+
+                    // Ensure the newly-refreshed anchors point to a valid excerpt and don't
+                    // overshoot its boundaries.
+                    assert_eq!(anchors.len(), prev_len);
+                    for anchor in &anchors {
+                        if anchor.excerpt_id == ExcerptId::min()
+                            || anchor.excerpt_id == ExcerptId::max()
+                        {
+                            continue;
+                        }
+
+                        let excerpt = multibuffer.excerpt(anchor.excerpt_id).unwrap();
+                        assert_eq!(excerpt.id, anchor.excerpt_id);
+                        assert!(excerpt.contains(anchor));
+                    }
+                }
+                _ => {
+                    let buffer_handle = if buffers.is_empty() || rng.gen_bool(0.4) {
+                        let base_text = util::RandomCharIter::new(&mut rng)
+                            .take(10)
+                            .collect::<String>();
+                        buffers.push(
+                            cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), base_text)),
+                        );
+                        buffers.last().unwrap()
+                    } else {
+                        buffers.choose(&mut rng).unwrap()
+                    };
+
+                    let buffer = buffer_handle.read(cx);
+                    let end_ix = buffer.clip_offset(rng.gen_range(0..=buffer.len()), Bias::Right);
+                    let start_ix = buffer.clip_offset(rng.gen_range(0..=end_ix), Bias::Left);
+                    let anchor_range = buffer.anchor_before(start_ix)..buffer.anchor_after(end_ix);
+                    let prev_excerpt_ix = rng.gen_range(0..=expected_excerpts.len());
+                    let prev_excerpt_id = excerpt_ids
+                        .get(prev_excerpt_ix)
+                        .cloned()
+                        .unwrap_or_else(ExcerptId::max);
+                    let excerpt_ix = (prev_excerpt_ix + 1).min(expected_excerpts.len());
+
+                    log::info!(
+                        "Inserting excerpt at {} of {} for buffer {}: {:?}[{:?}] = {:?}",
+                        excerpt_ix,
+                        expected_excerpts.len(),
+                        buffer_handle.read(cx).remote_id(),
+                        buffer.text(),
+                        start_ix..end_ix,
+                        &buffer.text()[start_ix..end_ix]
+                    );
+
+                    let excerpt_id = multibuffer.update(cx, |multibuffer, cx| {
+                        multibuffer
+                            .insert_excerpts_after(
+                                prev_excerpt_id,
+                                buffer_handle.clone(),
+                                [ExcerptRange {
+                                    context: start_ix..end_ix,
+                                    primary: None,
+                                }],
+                                cx,
+                            )
+                            .pop()
+                            .unwrap()
+                    });
+
+                    excerpt_ids.insert(excerpt_ix, excerpt_id);
+                    expected_excerpts.insert(excerpt_ix, (buffer_handle.clone(), anchor_range));
+                }
+            }
+
+            if rng.gen_bool(0.3) {
+                multibuffer.update(cx, |multibuffer, cx| {
+                    old_versions.push((multibuffer.snapshot(cx), multibuffer.subscribe()));
+                })
+            }
+
+            let snapshot = multibuffer.read(cx).snapshot(cx);
+
+            let mut excerpt_starts = Vec::new();
+            let mut expected_text = String::new();
+            let mut expected_buffer_rows = Vec::new();
+            for (buffer, range) in &expected_excerpts {
+                let buffer = buffer.read(cx);
+                let buffer_range = range.to_offset(buffer);
+
+                excerpt_starts.push(TextSummary::from(expected_text.as_str()));
+                expected_text.extend(buffer.text_for_range(buffer_range.clone()));
+                expected_text.push('\n');
+
+                let buffer_row_range = buffer.offset_to_point(buffer_range.start).row
+                    ..=buffer.offset_to_point(buffer_range.end).row;
+                for row in buffer_row_range {
+                    expected_buffer_rows.push(Some(row));
+                }
+            }
+            // Remove final trailing newline.
+            if !expected_excerpts.is_empty() {
+                expected_text.pop();
+            }
+
+            // Always report one buffer row
+            if expected_buffer_rows.is_empty() {
+                expected_buffer_rows.push(Some(0));
+            }
+
+            assert_eq!(snapshot.text(), expected_text);
+            log::info!("MultiBuffer text: {:?}", expected_text);
+
+            assert_eq!(
+                snapshot.buffer_rows(0).collect::<Vec<_>>(),
+                expected_buffer_rows,
+            );
+
+            for _ in 0..5 {
+                let start_row = rng.gen_range(0..=expected_buffer_rows.len());
+                assert_eq!(
+                    snapshot.buffer_rows(start_row as u32).collect::<Vec<_>>(),
+                    &expected_buffer_rows[start_row..],
+                    "buffer_rows({})",
+                    start_row
+                );
+            }
+
+            assert_eq!(
+                snapshot.max_buffer_row(),
+                expected_buffer_rows.into_iter().flatten().max().unwrap()
+            );
+
+            let mut excerpt_starts = excerpt_starts.into_iter();
+            for (buffer, range) in &expected_excerpts {
+                let buffer = buffer.read(cx);
+                let buffer_id = buffer.remote_id();
+                let buffer_range = range.to_offset(buffer);
+                let buffer_start_point = buffer.offset_to_point(buffer_range.start);
+                let buffer_start_point_utf16 =
+                    buffer.text_summary_for_range::<PointUtf16, _>(0..buffer_range.start);
+
+                let excerpt_start = excerpt_starts.next().unwrap();
+                let mut offset = excerpt_start.len;
+                let mut buffer_offset = buffer_range.start;
+                let mut point = excerpt_start.lines;
+                let mut buffer_point = buffer_start_point;
+                let mut point_utf16 = excerpt_start.lines_utf16();
+                let mut buffer_point_utf16 = buffer_start_point_utf16;
+                for ch in buffer
+                    .snapshot()
+                    .chunks(buffer_range.clone(), false)
+                    .flat_map(|c| c.text.chars())
+                {
+                    for _ in 0..ch.len_utf8() {
+                        let left_offset = snapshot.clip_offset(offset, Bias::Left);
+                        let right_offset = snapshot.clip_offset(offset, Bias::Right);
+                        let buffer_left_offset = buffer.clip_offset(buffer_offset, Bias::Left);
+                        let buffer_right_offset = buffer.clip_offset(buffer_offset, Bias::Right);
+                        assert_eq!(
+                            left_offset,
+                            excerpt_start.len + (buffer_left_offset - buffer_range.start),
+                            "clip_offset({:?}, Left). buffer: {:?}, buffer offset: {:?}",
+                            offset,
+                            buffer_id,
+                            buffer_offset,
+                        );
+                        assert_eq!(
+                            right_offset,
+                            excerpt_start.len + (buffer_right_offset - buffer_range.start),
+                            "clip_offset({:?}, Right). buffer: {:?}, buffer offset: {:?}",
+                            offset,
+                            buffer_id,
+                            buffer_offset,
+                        );
+
+                        let left_point = snapshot.clip_point(point, Bias::Left);
+                        let right_point = snapshot.clip_point(point, Bias::Right);
+                        let buffer_left_point = buffer.clip_point(buffer_point, Bias::Left);
+                        let buffer_right_point = buffer.clip_point(buffer_point, Bias::Right);
+                        assert_eq!(
+                            left_point,
+                            excerpt_start.lines + (buffer_left_point - buffer_start_point),
+                            "clip_point({:?}, Left). buffer: {:?}, buffer point: {:?}",
+                            point,
+                            buffer_id,
+                            buffer_point,
+                        );
+                        assert_eq!(
+                            right_point,
+                            excerpt_start.lines + (buffer_right_point - buffer_start_point),
+                            "clip_point({:?}, Right). buffer: {:?}, buffer point: {:?}",
+                            point,
+                            buffer_id,
+                            buffer_point,
+                        );
+
+                        assert_eq!(
+                            snapshot.point_to_offset(left_point),
+                            left_offset,
+                            "point_to_offset({:?})",
+                            left_point,
+                        );
+                        assert_eq!(
+                            snapshot.offset_to_point(left_offset),
+                            left_point,
+                            "offset_to_point({:?})",
+                            left_offset,
+                        );
+
+                        offset += 1;
+                        buffer_offset += 1;
+                        if ch == '\n' {
+                            point += Point::new(1, 0);
+                            buffer_point += Point::new(1, 0);
+                        } else {
+                            point += Point::new(0, 1);
+                            buffer_point += Point::new(0, 1);
+                        }
+                    }
+
+                    for _ in 0..ch.len_utf16() {
+                        let left_point_utf16 =
+                            snapshot.clip_point_utf16(Unclipped(point_utf16), Bias::Left);
+                        let right_point_utf16 =
+                            snapshot.clip_point_utf16(Unclipped(point_utf16), Bias::Right);
+                        let buffer_left_point_utf16 =
+                            buffer.clip_point_utf16(Unclipped(buffer_point_utf16), Bias::Left);
+                        let buffer_right_point_utf16 =
+                            buffer.clip_point_utf16(Unclipped(buffer_point_utf16), Bias::Right);
+                        assert_eq!(
+                            left_point_utf16,
+                            excerpt_start.lines_utf16()
+                                + (buffer_left_point_utf16 - buffer_start_point_utf16),
+                            "clip_point_utf16({:?}, Left). buffer: {:?}, buffer point_utf16: {:?}",
+                            point_utf16,
+                            buffer_id,
+                            buffer_point_utf16,
+                        );
+                        assert_eq!(
+                            right_point_utf16,
+                            excerpt_start.lines_utf16()
+                                + (buffer_right_point_utf16 - buffer_start_point_utf16),
+                            "clip_point_utf16({:?}, Right). buffer: {:?}, buffer point_utf16: {:?}",
+                            point_utf16,
+                            buffer_id,
+                            buffer_point_utf16,
+                        );
+
+                        if ch == '\n' {
+                            point_utf16 += PointUtf16::new(1, 0);
+                            buffer_point_utf16 += PointUtf16::new(1, 0);
+                        } else {
+                            point_utf16 += PointUtf16::new(0, 1);
+                            buffer_point_utf16 += PointUtf16::new(0, 1);
+                        }
+                    }
+                }
+            }
+
+            for (row, line) in expected_text.split('\n').enumerate() {
+                assert_eq!(
+                    snapshot.line_len(row as u32),
+                    line.len() as u32,
+                    "line_len({}).",
+                    row
+                );
+            }
+
+            let text_rope = Rope::from(expected_text.as_str());
+            for _ in 0..10 {
+                let end_ix = text_rope.clip_offset(rng.gen_range(0..=text_rope.len()), Bias::Right);
+                let start_ix = text_rope.clip_offset(rng.gen_range(0..=end_ix), Bias::Left);
+
+                let text_for_range = snapshot
+                    .text_for_range(start_ix..end_ix)
+                    .collect::<String>();
+                assert_eq!(
+                    text_for_range,
+                    &expected_text[start_ix..end_ix],
+                    "incorrect text for range {:?}",
+                    start_ix..end_ix
+                );
+
+                let excerpted_buffer_ranges = multibuffer
+                    .read(cx)
+                    .range_to_buffer_ranges(start_ix..end_ix, cx);
+                let excerpted_buffers_text = excerpted_buffer_ranges
+                    .iter()
+                    .map(|(buffer, buffer_range, _)| {
+                        buffer
+                            .read(cx)
+                            .text_for_range(buffer_range.clone())
+                            .collect::<String>()
+                    })
+                    .collect::<Vec<_>>()
+                    .join("\n");
+                assert_eq!(excerpted_buffers_text, text_for_range);
+                if !expected_excerpts.is_empty() {
+                    assert!(!excerpted_buffer_ranges.is_empty());
+                }
+
+                let expected_summary = TextSummary::from(&expected_text[start_ix..end_ix]);
+                assert_eq!(
+                    snapshot.text_summary_for_range::<TextSummary, _>(start_ix..end_ix),
+                    expected_summary,
+                    "incorrect summary for range {:?}",
+                    start_ix..end_ix
+                );
+            }
+
+            // Anchor resolution
+            let summaries = snapshot.summaries_for_anchors::<usize, _>(&anchors);
+            assert_eq!(anchors.len(), summaries.len());
+            for (anchor, resolved_offset) in anchors.iter().zip(summaries) {
+                assert!(resolved_offset <= snapshot.len());
+                assert_eq!(
+                    snapshot.summary_for_anchor::<usize>(anchor),
+                    resolved_offset
+                );
+            }
+
+            for _ in 0..10 {
+                let end_ix = text_rope.clip_offset(rng.gen_range(0..=text_rope.len()), Bias::Right);
+                assert_eq!(
+                    snapshot.reversed_chars_at(end_ix).collect::<String>(),
+                    expected_text[..end_ix].chars().rev().collect::<String>(),
+                );
+            }
+
+            for _ in 0..10 {
+                let end_ix = rng.gen_range(0..=text_rope.len());
+                let start_ix = rng.gen_range(0..=end_ix);
+                assert_eq!(
+                    snapshot
+                        .bytes_in_range(start_ix..end_ix)
+                        .flatten()
+                        .copied()
+                        .collect::<Vec<_>>(),
+                    expected_text.as_bytes()[start_ix..end_ix].to_vec(),
+                    "bytes_in_range({:?})",
+                    start_ix..end_ix,
+                );
+            }
+        }
+
+        let snapshot = multibuffer.read(cx).snapshot(cx);
+        for (old_snapshot, subscription) in old_versions {
+            let edits = subscription.consume().into_inner();
+
+            log::info!(
+                "applying subscription edits to old text: {:?}: {:?}",
+                old_snapshot.text(),
+                edits,
+            );
+
+            let mut text = old_snapshot.text();
+            for edit in edits {
+                let new_text: String = snapshot.text_for_range(edit.new.clone()).collect();
+                text.replace_range(edit.new.start..edit.new.start + edit.old.len(), &new_text);
+            }
+            assert_eq!(text.to_string(), snapshot.text());
+        }
+    }
+
+    #[gpui2::test]
+    fn test_history(cx: &mut AppContext) {
+        let test_settings = SettingsStore::test(cx);
+        cx.set_global(test_settings);
+
+        let buffer_1 = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "1234"));
+        let buffer_2 = cx.build_model(|cx| Buffer::new(0, cx.entity_id().as_u64(), "5678"));
+        let multibuffer = cx.build_model(|_| MultiBuffer::new(0));
+        let group_interval = multibuffer.read(cx).history.group_interval;
+        multibuffer.update(cx, |multibuffer, cx| {
+            multibuffer.push_excerpts(
+                buffer_1.clone(),
+                [ExcerptRange {
+                    context: 0..buffer_1.read(cx).len(),
+                    primary: None,
+                }],
+                cx,
+            );
+            multibuffer.push_excerpts(
+                buffer_2.clone(),
+                [ExcerptRange {
+                    context: 0..buffer_2.read(cx).len(),
+                    primary: None,
+                }],
+                cx,
+            );
+        });
+
+        let mut now = Instant::now();
+
+        multibuffer.update(cx, |multibuffer, cx| {
+            let transaction_1 = multibuffer.start_transaction_at(now, cx).unwrap();
+            multibuffer.edit(
+                [
+                    (Point::new(0, 0)..Point::new(0, 0), "A"),
+                    (Point::new(1, 0)..Point::new(1, 0), "A"),
+                ],
+                None,
+                cx,
+            );
+            multibuffer.edit(
+                [
+                    (Point::new(0, 1)..Point::new(0, 1), "B"),
+                    (Point::new(1, 1)..Point::new(1, 1), "B"),
+                ],
+                None,
+                cx,
+            );
+            multibuffer.end_transaction_at(now, cx);
+            assert_eq!(multibuffer.read(cx).text(), "AB1234\nAB5678");
+
+            // Edit buffer 1 through the multibuffer
+            now += 2 * group_interval;
+            multibuffer.start_transaction_at(now, cx);
+            multibuffer.edit([(2..2, "C")], None, cx);
+            multibuffer.end_transaction_at(now, cx);
+            assert_eq!(multibuffer.read(cx).text(), "ABC1234\nAB5678");
+
+            // Edit buffer 1 independently
+            buffer_1.update(cx, |buffer_1, cx| {
+                buffer_1.start_transaction_at(now);
+                buffer_1.edit([(3..3, "D")], None, cx);
+                buffer_1.end_transaction_at(now, cx);
+
+                now += 2 * group_interval;
+                buffer_1.start_transaction_at(now);
+                buffer_1.edit([(4..4, "E")], None, cx);
+                buffer_1.end_transaction_at(now, cx);
+            });
+            assert_eq!(multibuffer.read(cx).text(), "ABCDE1234\nAB5678");
+
+            // An undo in the multibuffer undoes the multibuffer transaction
+            // and also any individual buffer edits that have occurred since
+            // that transaction.
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "AB1234\nAB5678");
+
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "1234\n5678");
+
+            multibuffer.redo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "AB1234\nAB5678");
+
+            multibuffer.redo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "ABCDE1234\nAB5678");
+
+            // Undo buffer 2 independently.
+            buffer_2.update(cx, |buffer_2, cx| buffer_2.undo(cx));
+            assert_eq!(multibuffer.read(cx).text(), "ABCDE1234\n5678");
+
+            // An undo in the multibuffer undoes the components of the
+            // the last multibuffer transaction that are not already undone.
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "AB1234\n5678");
+
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "1234\n5678");
+
+            multibuffer.redo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "AB1234\nAB5678");
+
+            buffer_1.update(cx, |buffer_1, cx| buffer_1.redo(cx));
+            assert_eq!(multibuffer.read(cx).text(), "ABCD1234\nAB5678");
+
+            // Redo stack gets cleared after an edit.
+            now += 2 * group_interval;
+            multibuffer.start_transaction_at(now, cx);
+            multibuffer.edit([(0..0, "X")], None, cx);
+            multibuffer.end_transaction_at(now, cx);
+            assert_eq!(multibuffer.read(cx).text(), "XABCD1234\nAB5678");
+            multibuffer.redo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "XABCD1234\nAB5678");
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "ABCD1234\nAB5678");
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "1234\n5678");
+
+            // Transactions can be grouped manually.
+            multibuffer.redo(cx);
+            multibuffer.redo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "XABCD1234\nAB5678");
+            multibuffer.group_until_transaction(transaction_1, cx);
+            multibuffer.undo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "1234\n5678");
+            multibuffer.redo(cx);
+            assert_eq!(multibuffer.read(cx).text(), "XABCD1234\nAB5678");
+        });
+    }
+}