uniform_list.rs

  1//! A scrollable list of elements with uniform height, optimized for large lists.
  2//! Rather than use the full taffy layout system, uniform_list simply measures
  3//! the first element and then lays out all remaining elements in a line based on that
  4//! measurement. This is much faster than the full layout system, but only works for
  5//! elements with uniform height.
  6
  7use crate::{
  8    AnyElement, App, AvailableSpace, Bounds, ContentMask, Element, ElementId, Entity,
  9    GlobalElementId, Hitbox, InspectorElementId, InteractiveElement, Interactivity, IntoElement,
 10    IsZero, LayoutId, ListSizingBehavior, Overflow, Pixels, Point, ScrollHandle, Size,
 11    StyleRefinement, Styled, Window, point, size,
 12};
 13use smallvec::SmallVec;
 14use std::{cell::RefCell, cmp, ops::Range, rc::Rc};
 15
 16use super::ListHorizontalSizingBehavior;
 17
 18/// uniform_list provides lazy rendering for a set of items that are of uniform height.
 19/// When rendered into a container with overflow-y: hidden and a fixed (or max) height,
 20/// uniform_list will only render the visible subset of items.
 21#[track_caller]
 22pub fn uniform_list<R>(
 23    id: impl Into<ElementId>,
 24    item_count: usize,
 25    f: impl 'static + Fn(Range<usize>, &mut Window, &mut App) -> Vec<R>,
 26) -> UniformList
 27where
 28    R: IntoElement,
 29{
 30    let id = id.into();
 31    let mut base_style = StyleRefinement::default();
 32    base_style.overflow.y = Some(Overflow::Scroll);
 33
 34    let render_range = move |range: Range<usize>, window: &mut Window, cx: &mut App| {
 35        f(range, window, cx)
 36            .into_iter()
 37            .map(|component| component.into_any_element())
 38            .collect()
 39    };
 40
 41    UniformList {
 42        item_count,
 43        item_to_measure_index: 0,
 44        render_items: Box::new(render_range),
 45        decorations: Vec::new(),
 46        interactivity: Interactivity {
 47            element_id: Some(id),
 48            base_style: Box::new(base_style),
 49            ..Interactivity::new()
 50        },
 51        scroll_handle: None,
 52        sizing_behavior: ListSizingBehavior::default(),
 53        horizontal_sizing_behavior: ListHorizontalSizingBehavior::default(),
 54    }
 55}
 56
 57/// A list element for efficiently laying out and displaying a list of uniform-height elements.
 58pub struct UniformList {
 59    item_count: usize,
 60    item_to_measure_index: usize,
 61    render_items: Box<
 62        dyn for<'a> Fn(Range<usize>, &'a mut Window, &'a mut App) -> SmallVec<[AnyElement; 64]>,
 63    >,
 64    decorations: Vec<Box<dyn UniformListDecoration>>,
 65    interactivity: Interactivity,
 66    scroll_handle: Option<UniformListScrollHandle>,
 67    sizing_behavior: ListSizingBehavior,
 68    horizontal_sizing_behavior: ListHorizontalSizingBehavior,
 69}
 70
 71/// Frame state used by the [UniformList].
 72pub struct UniformListFrameState {
 73    items: SmallVec<[AnyElement; 32]>,
 74    decorations: SmallVec<[AnyElement; 2]>,
 75}
 76
 77/// A handle for controlling the scroll position of a uniform list.
 78/// This should be stored in your view and passed to the uniform_list on each frame.
 79#[derive(Clone, Debug, Default)]
 80pub struct UniformListScrollHandle(pub Rc<RefCell<UniformListScrollState>>);
 81
 82/// Where to place the element scrolled to.
 83#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 84pub enum ScrollStrategy {
 85    /// Place the element at the top of the list's viewport.
 86    Top,
 87    /// Attempt to place the element in the middle of the list's viewport.
 88    /// May not be possible if there's not enough list items above the item scrolled to:
 89    /// in this case, the element will be placed at the closest possible position.
 90    Center,
 91    /// Attempt to place the element at the bottom of the list's viewport.
 92    /// May not be possible if there's not enough list items above the item scrolled to:
 93    /// in this case, the element will be placed at the closest possible position.
 94    Bottom,
 95}
 96
 97#[derive(Clone, Copy, Debug)]
 98#[allow(missing_docs)]
 99pub struct DeferredScrollToItem {
100    /// The item index to scroll to
101    pub item_index: usize,
102    /// The scroll strategy to use
103    pub strategy: ScrollStrategy,
104    /// The offset in number of items
105    pub offset: usize,
106    pub scroll_strict: bool,
107}
108
109#[derive(Clone, Debug, Default)]
110#[allow(missing_docs)]
111pub struct UniformListScrollState {
112    pub base_handle: ScrollHandle,
113    pub deferred_scroll_to_item: Option<DeferredScrollToItem>,
114    /// Size of the item, captured during last layout.
115    pub last_item_size: Option<ItemSize>,
116    /// Whether the list was vertically flipped during last layout.
117    pub y_flipped: bool,
118}
119
120#[derive(Copy, Clone, Debug, Default)]
121/// The size of the item and its contents.
122pub struct ItemSize {
123    /// The size of the item.
124    pub item: Size<Pixels>,
125    /// The size of the item's contents, which may be larger than the item itself,
126    /// if the item was bounded by a parent element.
127    pub contents: Size<Pixels>,
128}
129
130impl UniformListScrollHandle {
131    /// Create a new scroll handle to bind to a uniform list.
132    pub fn new() -> Self {
133        Self(Rc::new(RefCell::new(UniformListScrollState {
134            base_handle: ScrollHandle::new(),
135            deferred_scroll_to_item: None,
136            last_item_size: None,
137            y_flipped: false,
138        })))
139    }
140
141    /// Scroll the list so that the given item index is visible.
142    ///
143    /// This uses non-strict scrolling: if the item is already fully visible, no scrolling occurs.
144    /// If the item is out of view, it scrolls the minimum amount to bring it into view according
145    /// to the strategy.
146    pub fn scroll_to_item(&self, ix: usize, strategy: ScrollStrategy) {
147        self.0.borrow_mut().deferred_scroll_to_item = Some(DeferredScrollToItem {
148            item_index: ix,
149            strategy,
150            offset: 0,
151            scroll_strict: false,
152        });
153    }
154
155    /// Scroll the list so that the given item index is at scroll strategy position.
156    ///
157    /// This uses strict scrolling: the item will always be scrolled to match the strategy position,
158    /// even if it's already visible. Use this when you need precise positioning.
159    pub fn scroll_to_item_strict(&self, ix: usize, strategy: ScrollStrategy) {
160        self.0.borrow_mut().deferred_scroll_to_item = Some(DeferredScrollToItem {
161            item_index: ix,
162            strategy,
163            offset: 0,
164            scroll_strict: true,
165        });
166    }
167
168    /// Scroll the list to the given item index with an offset in number of items.
169    ///
170    /// This uses non-strict scrolling: if the item is already visible within the offset region,
171    /// no scrolling occurs.
172    ///
173    /// The offset parameter shrinks the effective viewport by the specified number of items
174    /// from the corresponding edge, then applies the scroll strategy within that reduced viewport:
175    /// - `ScrollStrategy::Top`: Shrinks from top, positions item at the new top
176    /// - `ScrollStrategy::Center`: Shrinks from top, centers item in the reduced viewport
177    /// - `ScrollStrategy::Bottom`: Shrinks from bottom, positions item at the new bottom
178    pub fn scroll_to_item_with_offset(&self, ix: usize, strategy: ScrollStrategy, offset: usize) {
179        self.0.borrow_mut().deferred_scroll_to_item = Some(DeferredScrollToItem {
180            item_index: ix,
181            strategy,
182            offset,
183            scroll_strict: false,
184        });
185    }
186
187    /// Scroll the list so that the given item index is at the exact scroll strategy position with an offset.
188    ///
189    /// This uses strict scrolling: the item will always be scrolled to match the strategy position,
190    /// even if it's already visible.
191    ///
192    /// The offset parameter shrinks the effective viewport by the specified number of items
193    /// from the corresponding edge, then applies the scroll strategy within that reduced viewport:
194    /// - `ScrollStrategy::Top`: Shrinks from top, positions item at the new top
195    /// - `ScrollStrategy::Center`: Shrinks from top, centers item in the reduced viewport
196    /// - `ScrollStrategy::Bottom`: Shrinks from bottom, positions item at the new bottom
197    pub fn scroll_to_item_strict_with_offset(
198        &self,
199        ix: usize,
200        strategy: ScrollStrategy,
201        offset: usize,
202    ) {
203        self.0.borrow_mut().deferred_scroll_to_item = Some(DeferredScrollToItem {
204            item_index: ix,
205            strategy,
206            offset,
207            scroll_strict: true,
208        });
209    }
210
211    /// Check if the list is flipped vertically.
212    pub fn y_flipped(&self) -> bool {
213        self.0.borrow().y_flipped
214    }
215
216    /// Get the index of the topmost visible child.
217    #[cfg(any(test, feature = "test-support"))]
218    pub fn logical_scroll_top_index(&self) -> usize {
219        let this = self.0.borrow();
220        this.deferred_scroll_to_item
221            .as_ref()
222            .map(|deferred| deferred.item_index)
223            .unwrap_or_else(|| this.base_handle.logical_scroll_top().0)
224    }
225
226    /// Checks if the list can be scrolled vertically.
227    pub fn is_scrollable(&self) -> bool {
228        if let Some(size) = self.0.borrow().last_item_size {
229            size.contents.height > size.item.height
230        } else {
231            false
232        }
233    }
234}
235
236impl Styled for UniformList {
237    fn style(&mut self) -> &mut StyleRefinement {
238        &mut self.interactivity.base_style
239    }
240}
241
242impl Element for UniformList {
243    type RequestLayoutState = UniformListFrameState;
244    type PrepaintState = Option<Hitbox>;
245
246    fn id(&self) -> Option<ElementId> {
247        self.interactivity.element_id.clone()
248    }
249
250    fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
251        None
252    }
253
254    fn request_layout(
255        &mut self,
256        global_id: Option<&GlobalElementId>,
257        inspector_id: Option<&InspectorElementId>,
258        window: &mut Window,
259        cx: &mut App,
260    ) -> (LayoutId, Self::RequestLayoutState) {
261        let max_items = self.item_count;
262        let item_size = self.measure_item(None, window, cx);
263        let layout_id = self.interactivity.request_layout(
264            global_id,
265            inspector_id,
266            window,
267            cx,
268            |style, window, cx| match self.sizing_behavior {
269                ListSizingBehavior::Infer => {
270                    window.with_text_style(style.text_style().cloned(), |window| {
271                        window.request_measured_layout(
272                            style,
273                            move |known_dimensions, available_space, _window, _cx| {
274                                let desired_height = item_size.height * max_items;
275                                let width = known_dimensions.width.unwrap_or(match available_space
276                                    .width
277                                {
278                                    AvailableSpace::Definite(x) => x,
279                                    AvailableSpace::MinContent | AvailableSpace::MaxContent => {
280                                        item_size.width
281                                    }
282                                });
283                                let height = match available_space.height {
284                                    AvailableSpace::Definite(height) => desired_height.min(height),
285                                    AvailableSpace::MinContent | AvailableSpace::MaxContent => {
286                                        desired_height
287                                    }
288                                };
289                                size(width, height)
290                            },
291                        )
292                    })
293                }
294                ListSizingBehavior::Auto => window
295                    .with_text_style(style.text_style().cloned(), |window| {
296                        window.request_layout(style, None, cx)
297                    }),
298            },
299        );
300
301        (
302            layout_id,
303            UniformListFrameState {
304                items: SmallVec::new(),
305                decorations: SmallVec::new(),
306            },
307        )
308    }
309
310    fn prepaint(
311        &mut self,
312        global_id: Option<&GlobalElementId>,
313        inspector_id: Option<&InspectorElementId>,
314        bounds: Bounds<Pixels>,
315        frame_state: &mut Self::RequestLayoutState,
316        window: &mut Window,
317        cx: &mut App,
318    ) -> Option<Hitbox> {
319        let style = self
320            .interactivity
321            .compute_style(global_id, None, window, cx);
322        let border = style.border_widths.to_pixels(window.rem_size());
323        let padding = style
324            .padding
325            .to_pixels(bounds.size.into(), window.rem_size());
326
327        let padded_bounds = Bounds::from_corners(
328            bounds.origin + point(border.left + padding.left, border.top + padding.top),
329            bounds.bottom_right()
330                - point(border.right + padding.right, border.bottom + padding.bottom),
331        );
332
333        let can_scroll_horizontally = matches!(
334            self.horizontal_sizing_behavior,
335            ListHorizontalSizingBehavior::Unconstrained
336        );
337
338        let longest_item_size = self.measure_item(None, window, cx);
339        let content_width = if can_scroll_horizontally {
340            padded_bounds.size.width.max(longest_item_size.width)
341        } else {
342            padded_bounds.size.width
343        };
344        let content_size = Size {
345            width: content_width,
346            height: longest_item_size.height * self.item_count + padding.top + padding.bottom,
347        };
348
349        let shared_scroll_offset = self.interactivity.scroll_offset.clone().unwrap();
350        let item_height = longest_item_size.height;
351        let shared_scroll_to_item = self.scroll_handle.as_mut().and_then(|handle| {
352            let mut handle = handle.0.borrow_mut();
353            handle.last_item_size = Some(ItemSize {
354                item: padded_bounds.size,
355                contents: content_size,
356            });
357            handle.deferred_scroll_to_item.take()
358        });
359
360        self.interactivity.prepaint(
361            global_id,
362            inspector_id,
363            bounds,
364            content_size,
365            window,
366            cx,
367            |_style, mut scroll_offset, hitbox, window, cx| {
368                let y_flipped = if let Some(scroll_handle) = &self.scroll_handle {
369                    let scroll_state = scroll_handle.0.borrow();
370                    scroll_state.y_flipped
371                } else {
372                    false
373                };
374
375                if self.item_count > 0 {
376                    let content_height =
377                        item_height * self.item_count + padding.top + padding.bottom;
378                    let is_scrolled_vertically = !scroll_offset.y.is_zero();
379                    let min_vertical_scroll_offset = padded_bounds.size.height - content_height;
380                    if is_scrolled_vertically && scroll_offset.y < min_vertical_scroll_offset {
381                        shared_scroll_offset.borrow_mut().y = min_vertical_scroll_offset;
382                        scroll_offset.y = min_vertical_scroll_offset;
383                    }
384
385                    let content_width = content_size.width + padding.left + padding.right;
386                    let is_scrolled_horizontally =
387                        can_scroll_horizontally && !scroll_offset.x.is_zero();
388                    if is_scrolled_horizontally && content_width <= padded_bounds.size.width {
389                        shared_scroll_offset.borrow_mut().x = Pixels::ZERO;
390                        scroll_offset.x = Pixels::ZERO;
391                    }
392
393                    if let Some(deferred_scroll) = shared_scroll_to_item {
394                        let mut ix = deferred_scroll.item_index;
395                        if y_flipped {
396                            ix = self.item_count.saturating_sub(ix + 1);
397                        }
398                        let list_height = padded_bounds.size.height;
399                        let mut updated_scroll_offset = shared_scroll_offset.borrow_mut();
400                        let item_top = item_height * ix + padding.top;
401                        let item_bottom = item_top + item_height;
402                        let scroll_top = -updated_scroll_offset.y;
403                        let offset_pixels = item_height * deferred_scroll.offset;
404                        let mut scrolled_to_top = false;
405
406                        if item_top < scroll_top + padding.top + offset_pixels {
407                            scrolled_to_top = true;
408                            updated_scroll_offset.y = -(item_top) + padding.top + offset_pixels;
409                        } else if item_bottom > scroll_top + list_height - padding.bottom {
410                            scrolled_to_top = true;
411                            updated_scroll_offset.y = -(item_bottom - list_height) - padding.bottom;
412                        }
413
414                        if deferred_scroll.scroll_strict
415                            || (scrolled_to_top
416                                && (item_top < scroll_top + offset_pixels
417                                    || item_bottom > scroll_top + list_height))
418                        {
419                            match deferred_scroll.strategy {
420                                ScrollStrategy::Top => {
421                                    updated_scroll_offset.y = -(item_top - offset_pixels)
422                                        .max(Pixels::ZERO)
423                                        .min(content_height - list_height)
424                                        .max(Pixels::ZERO);
425                                }
426                                ScrollStrategy::Center => {
427                                    let item_center = item_top + item_height / 2.0;
428
429                                    let viewport_height = list_height - offset_pixels;
430                                    let viewport_center = offset_pixels + viewport_height / 2.0;
431                                    let target_scroll_top = item_center - viewport_center;
432
433                                    updated_scroll_offset.y = -target_scroll_top
434                                        .max(Pixels::ZERO)
435                                        .min(content_height - list_height)
436                                        .max(Pixels::ZERO);
437                                }
438                                ScrollStrategy::Bottom => {
439                                    updated_scroll_offset.y = -(item_bottom - list_height
440                                        + offset_pixels)
441                                        .max(Pixels::ZERO)
442                                        .min(content_height - list_height)
443                                        .max(Pixels::ZERO);
444                                }
445                            }
446                        }
447                        scroll_offset = *updated_scroll_offset
448                    }
449
450                    let first_visible_element_ix =
451                        (-(scroll_offset.y + padding.top) / item_height).floor() as usize;
452                    let last_visible_element_ix = ((-scroll_offset.y + padded_bounds.size.height)
453                        / item_height)
454                        .ceil() as usize;
455
456                    let visible_range = first_visible_element_ix
457                        ..cmp::min(last_visible_element_ix, self.item_count);
458
459                    let items = if y_flipped {
460                        let flipped_range = self.item_count.saturating_sub(visible_range.end)
461                            ..self.item_count.saturating_sub(visible_range.start);
462                        let mut items = (self.render_items)(flipped_range, window, cx);
463                        items.reverse();
464                        items
465                    } else {
466                        (self.render_items)(visible_range.clone(), window, cx)
467                    };
468
469                    let content_mask = ContentMask { bounds };
470                    window.with_content_mask(Some(content_mask), |window| {
471                        for (mut item, ix) in items.into_iter().zip(visible_range.clone()) {
472                            let item_origin = padded_bounds.origin
473                                + point(
474                                    if can_scroll_horizontally {
475                                        scroll_offset.x + padding.left
476                                    } else {
477                                        scroll_offset.x
478                                    },
479                                    item_height * ix + scroll_offset.y + padding.top,
480                                );
481                            let available_width = if can_scroll_horizontally {
482                                padded_bounds.size.width + scroll_offset.x.abs()
483                            } else {
484                                padded_bounds.size.width
485                            };
486                            let available_space = size(
487                                AvailableSpace::Definite(available_width),
488                                AvailableSpace::Definite(item_height),
489                            );
490                            item.layout_as_root(available_space, window, cx);
491                            item.prepaint_at(item_origin, window, cx);
492                            frame_state.items.push(item);
493                        }
494
495                        let bounds = Bounds::new(
496                            padded_bounds.origin
497                                + point(
498                                    if can_scroll_horizontally {
499                                        scroll_offset.x + padding.left
500                                    } else {
501                                        scroll_offset.x
502                                    },
503                                    scroll_offset.y + padding.top,
504                                ),
505                            padded_bounds.size,
506                        );
507                        for decoration in &self.decorations {
508                            let mut decoration = decoration.as_ref().compute(
509                                visible_range.clone(),
510                                bounds,
511                                scroll_offset,
512                                item_height,
513                                self.item_count,
514                                window,
515                                cx,
516                            );
517                            let available_space = size(
518                                AvailableSpace::Definite(bounds.size.width),
519                                AvailableSpace::Definite(bounds.size.height),
520                            );
521                            decoration.layout_as_root(available_space, window, cx);
522                            decoration.prepaint_at(bounds.origin, window, cx);
523                            frame_state.decorations.push(decoration);
524                        }
525                    });
526                }
527
528                hitbox
529            },
530        )
531    }
532
533    fn paint(
534        &mut self,
535        global_id: Option<&GlobalElementId>,
536        inspector_id: Option<&InspectorElementId>,
537        bounds: Bounds<crate::Pixels>,
538        request_layout: &mut Self::RequestLayoutState,
539        hitbox: &mut Option<Hitbox>,
540        window: &mut Window,
541        cx: &mut App,
542    ) {
543        self.interactivity.paint(
544            global_id,
545            inspector_id,
546            bounds,
547            hitbox.as_ref(),
548            window,
549            cx,
550            |_, window, cx| {
551                for item in &mut request_layout.items {
552                    item.paint(window, cx);
553                }
554                for decoration in &mut request_layout.decorations {
555                    decoration.paint(window, cx);
556                }
557            },
558        )
559    }
560}
561
562impl IntoElement for UniformList {
563    type Element = Self;
564
565    fn into_element(self) -> Self::Element {
566        self
567    }
568}
569
570/// A decoration for a [`UniformList`]. This can be used for various things,
571/// such as rendering indent guides, or other visual effects.
572pub trait UniformListDecoration {
573    /// Compute the decoration element, given the visible range of list items,
574    /// the bounds of the list, and the height of each item.
575    fn compute(
576        &self,
577        visible_range: Range<usize>,
578        bounds: Bounds<Pixels>,
579        scroll_offset: Point<Pixels>,
580        item_height: Pixels,
581        item_count: usize,
582        window: &mut Window,
583        cx: &mut App,
584    ) -> AnyElement;
585}
586
587impl<T: UniformListDecoration + 'static> UniformListDecoration for Entity<T> {
588    fn compute(
589        &self,
590        visible_range: Range<usize>,
591        bounds: Bounds<Pixels>,
592        scroll_offset: Point<Pixels>,
593        item_height: Pixels,
594        item_count: usize,
595        window: &mut Window,
596        cx: &mut App,
597    ) -> AnyElement {
598        self.update(cx, |inner, cx| {
599            inner.compute(
600                visible_range,
601                bounds,
602                scroll_offset,
603                item_height,
604                item_count,
605                window,
606                cx,
607            )
608        })
609    }
610}
611
612impl UniformList {
613    /// Selects a specific list item for measurement.
614    pub fn with_width_from_item(mut self, item_index: Option<usize>) -> Self {
615        self.item_to_measure_index = item_index.unwrap_or(0);
616        self
617    }
618
619    /// Sets the sizing behavior, similar to the `List` element.
620    pub fn with_sizing_behavior(mut self, behavior: ListSizingBehavior) -> Self {
621        self.sizing_behavior = behavior;
622        self
623    }
624
625    /// Sets the horizontal sizing behavior, controlling the way list items laid out horizontally.
626    /// With [`ListHorizontalSizingBehavior::Unconstrained`] behavior, every item and the list itself will
627    /// have the size of the widest item and lay out pushing the `end_slot` to the right end.
628    pub fn with_horizontal_sizing_behavior(
629        mut self,
630        behavior: ListHorizontalSizingBehavior,
631    ) -> Self {
632        self.horizontal_sizing_behavior = behavior;
633        match behavior {
634            ListHorizontalSizingBehavior::FitList => {
635                self.interactivity.base_style.overflow.x = None;
636            }
637            ListHorizontalSizingBehavior::Unconstrained => {
638                self.interactivity.base_style.overflow.x = Some(Overflow::Scroll);
639            }
640        }
641        self
642    }
643
644    /// Adds a decoration element to the list.
645    pub fn with_decoration(mut self, decoration: impl UniformListDecoration + 'static) -> Self {
646        self.decorations.push(Box::new(decoration));
647        self
648    }
649
650    fn measure_item(
651        &self,
652        list_width: Option<Pixels>,
653        window: &mut Window,
654        cx: &mut App,
655    ) -> Size<Pixels> {
656        if self.item_count == 0 {
657            return Size::default();
658        }
659
660        let item_ix = cmp::min(self.item_to_measure_index, self.item_count - 1);
661        let mut items = (self.render_items)(item_ix..item_ix + 1, window, cx);
662        let Some(mut item_to_measure) = items.pop() else {
663            return Size::default();
664        };
665        let available_space = size(
666            list_width.map_or(AvailableSpace::MinContent, |width| {
667                AvailableSpace::Definite(width)
668            }),
669            AvailableSpace::MinContent,
670        );
671        item_to_measure.layout_as_root(available_space, window, cx)
672    }
673
674    /// Track and render scroll state of this list with reference to the given scroll handle.
675    pub fn track_scroll(mut self, handle: UniformListScrollHandle) -> Self {
676        self.interactivity.tracked_scroll_handle = Some(handle.0.borrow().base_handle.clone());
677        self.scroll_handle = Some(handle);
678        self
679    }
680
681    /// Sets whether the list is flipped vertically, such that item 0 appears at the bottom.
682    pub fn y_flipped(mut self, y_flipped: bool) -> Self {
683        if let Some(ref scroll_handle) = self.scroll_handle {
684            let mut scroll_state = scroll_handle.0.borrow_mut();
685            let mut base_handle = &scroll_state.base_handle;
686            let offset = base_handle.offset();
687            match scroll_state.last_item_size {
688                Some(last_size) if scroll_state.y_flipped != y_flipped => {
689                    let new_y_offset =
690                        -(offset.y + last_size.contents.height - last_size.item.height);
691                    base_handle.set_offset(point(offset.x, new_y_offset));
692                    scroll_state.y_flipped = y_flipped;
693                }
694                // Handle case where list is initially flipped.
695                None if y_flipped => {
696                    base_handle.set_offset(point(offset.x, Pixels::MIN));
697                    scroll_state.y_flipped = y_flipped;
698                }
699                _ => {}
700            }
701        }
702        self
703    }
704}
705
706impl InteractiveElement for UniformList {
707    fn interactivity(&mut self) -> &mut crate::Interactivity {
708        &mut self.interactivity
709    }
710}