From 6d4915df16990bd04e3eee47e3b1dd18a0b35bd3 Mon Sep 17 00:00:00 2001
From: Nathan Sobo <nathan@zed.dev>
Date: Tue, 16 Dec 2025 10:58:10 -0700
Subject: [PATCH] Revert "Optimize editor rendering when clipped by parent
 containers" (#45011)
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This reverts commit 914b0117fb5a23469af85e567d5723eca6b53635 (#44995).

The optimization introduced a regression that causes the main thread to
hang for **100+ seconds** in certain scenarios, requiring a force quit
to recover.

## Analysis from spindump

When a large `AutoHeight` editor is displayed inside a `List` (e.g.,
Agent Panel thread view), the clipping calculation can produce invalid
row ranges:

1. `visible_bounds` from `window.content_mask().bounds` represents the
window's content mask, not the intersection with the editor
2. When the editor is partially scrolled out of view,
`clipped_top_in_lines` becomes extremely large
3. This causes `start_row` to be computed as an astronomically high
value
4. `blocks_in_range(start_row..end_row)` then spends excessive time in
`Cursor::search_forward` iterating through the block tree

The spindump showed **~46% of samples** (459/1001 over 10+ seconds)
stuck in `BlockSnapshot::blocks_in_range()`, specifically in cursor
iteration.

### Heaviest stack trace
```
EditorElement::prepaint
  └─ blocks_in_range + 236
       └─ Cursor::search_forward (459 samples)
```

## Symptoms

- Main thread unresponsive for 33-113 seconds before sampling even began
- UI completely frozen
- High CPU usage on main thread (10+ seconds of CPU time in the sample)
- Force quit required to recover

## Path forward

The original optimization goal (reducing line layout work for clipped
editors) is valid, but the implementation needs to:
1. Correctly calculate the **intersection** of editor bounds with the
visible viewport
2. Ensure row calculations stay within valid ranges (clamped to
`max_row`)
3. Handle edge cases where the editor is completely outside the visible
bounds

Release Notes:

- Fixed a hang that could occur when viewing large diffs in the Agent
Panel
---
 crates/editor/src/element.rs | 17 ++---------------
 crates/editor/src/test.rs    |  8 +++++---
 2 files changed, 7 insertions(+), 18 deletions(-)

diff --git a/crates/editor/src/element.rs b/crates/editor/src/element.rs
index 95047569c31c2e306b0f984832b568f052e4179a..8de660275ba9b455aec610568c41347888654495 100644
--- a/crates/editor/src/element.rs
+++ b/crates/editor/src/element.rs
@@ -9164,15 +9164,6 @@ impl Element for EditorElement {
                     let height_in_lines = f64::from(bounds.size.height / line_height);
                     let max_row = snapshot.max_point().row().as_f64();
 
-                    // Calculate how much of the editor is clipped by parent containers (e.g., List).
-                    // This allows us to only render lines that are actually visible, which is
-                    // critical for performance when large AutoHeight editors are inside Lists.
-                    let visible_bounds = window.content_mask().bounds;
-                    let clipped_top = (visible_bounds.origin.y - bounds.origin.y).max(px(0.));
-                    let clipped_top_in_lines = f64::from(clipped_top / line_height);
-                    let visible_height_in_lines =
-                        f64::from(visible_bounds.size.height / line_height);
-
                     // The max scroll position for the top of the window
                     let max_scroll_top = if matches!(
                         snapshot.mode,
@@ -9229,14 +9220,10 @@ impl Element for EditorElement {
                     let mut scroll_position = snapshot.scroll_position();
                     // The scroll position is a fractional point, the whole number of which represents
                     // the top of the window in terms of display rows.
-                    // We add clipped_top_in_lines to skip rows that are clipped by parent containers,
-                    // but we don't modify scroll_position itself since the parent handles positioning.
-                    let start_row =
-                        DisplayRow((scroll_position.y + clipped_top_in_lines).floor() as u32);
+                    let start_row = DisplayRow(scroll_position.y as u32);
                     let max_row = snapshot.max_point().row();
                     let end_row = cmp::min(
-                        (scroll_position.y + clipped_top_in_lines + visible_height_in_lines).ceil()
-                            as u32,
+                        (scroll_position.y + height_in_lines).ceil() as u32,
                         max_row.next_row().0,
                     );
                     let end_row = DisplayRow(end_row);
diff --git a/crates/editor/src/test.rs b/crates/editor/src/test.rs
index 1cc619385446502db6a3a0dceb6e70fa4b4e8416..5a0652bdd199a638f92234b1d50232071db18e07 100644
--- a/crates/editor/src/test.rs
+++ b/crates/editor/src/test.rs
@@ -176,9 +176,11 @@ pub fn block_content_for_tests(
 }
 
 pub fn editor_content_with_blocks(editor: &Entity<Editor>, cx: &mut VisualTestContext) -> String {
-    let draw_size = size(px(3000.0), px(3000.0));
-    cx.simulate_resize(draw_size);
-    cx.draw(gpui::Point::default(), draw_size, |_, _| editor.clone());
+    cx.draw(
+        gpui::Point::default(),
+        size(px(3000.0), px(3000.0)),
+        |_, _| editor.clone(),
+    );
     let (snapshot, mut lines, blocks) = editor.update_in(cx, |editor, window, cx| {
         let snapshot = editor.snapshot(window, cx);
         let text = editor.display_text(cx);