gpui_core/elements/

list.rs

//! A list element that can be used to render a large number of differently sized elements
//! efficiently. Clients of this API need to ensure that elements outside of the scrolled
//! area do not change their height for this element to function correctly. If your elements
//! do change height, notify the list element via [`ListState::splice`] or [`ListState::reset`].
//! In order to minimize re-renders, this element's state is stored intrusively
//! on your own views, so that your code can coordinate directly with the list element's cached state.
//!
//! If all of your elements are the same height, see [`crate::UniformList`] for a simpler API

use crate::{
    AnyElement, App, AvailableSpace, Bounds, ContentMask, DispatchPhase, Edges, Element, EntityId,
    FocusHandle, GlobalElementId, Hitbox, HitboxBehavior, InspectorElementId, IntoElement,
    Overflow, Pixels, Point, ScrollDelta, ScrollWheelEvent, Size, Style, StyleRefinement, Styled,
    Window, point, px, size,
};
use collections::VecDeque;
use refineable::Refineable as _;
use std::{cell::RefCell, ops::Range, rc::Rc};
use sum_tree::{Bias, Dimensions, SumTree};

type RenderItemFn = dyn FnMut(usize, &mut Window, &mut App) -> AnyElement + 'static;

/// Construct a new list element
pub fn list(
    state: ListState,
    render_item: impl FnMut(usize, &mut Window, &mut App) -> AnyElement + 'static,
) -> List {
    List {
        state,
        render_item: Box::new(render_item),
        style: StyleRefinement::default(),
        sizing_behavior: ListSizingBehavior::default(),
    }
}

/// A list element
pub struct List {
    state: ListState,
    render_item: Box<RenderItemFn>,
    style: StyleRefinement,
    sizing_behavior: ListSizingBehavior,
}

impl List {
    /// Set the sizing behavior for the list.
    pub fn with_sizing_behavior(mut self, behavior: ListSizingBehavior) -> Self {
        self.sizing_behavior = behavior;
        self
    }
}

/// The list state that views must hold on behalf of the list element.
#[derive(Clone)]
pub struct ListState(Rc<RefCell<StateInner>>);

impl std::fmt::Debug for ListState {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str("ListState")
    }
}

struct StateInner {
    last_layout_bounds: Option<Bounds<Pixels>>,
    last_padding: Option<Edges<Pixels>>,
    items: SumTree<ListItem>,
    logical_scroll_top: Option<ListOffset>,
    alignment: ListAlignment,
    overdraw: Pixels,
    reset: bool,
    #[allow(clippy::type_complexity)]
    scroll_handler: Option<Box<dyn FnMut(&ListScrollEvent, &mut Window, &mut App)>>,
    scrollbar_drag_start_height: Option<Pixels>,
    measuring_behavior: ListMeasuringBehavior,
    pending_scroll: Option<PendingScroll>,
    follow_state: FollowState,
}

/// Deferred scroll adjustment applied after the scroll-top item has been remeasured.
#[derive(Clone)]
enum PendingScroll {
    /// Preserve the same pixel offset into the item after it is remeasured.
    Absolute { item_ix: usize, offset: Pixels },
    /// Preserve the same fractional offset into the item after it is remeasured.
    Proportional(PendingScrollFraction),
}

/// Keeps track of a fractional scroll position within an item for restoration
/// after remeasurement.
#[derive(Clone)]
struct PendingScrollFraction {
    item_ix: usize,
    fraction: f32,
}

enum ScrollAnchor {
    Absolute,
    Proportional,
}

/// Controls whether the list automatically follows new content at the end.
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub enum FollowMode {
    /// Normal scrolling; no automatic following.
    #[default]
    Normal,
    /// Auto-scroll along with the tail when scrolled to bottom.
    Tail,
}

#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
enum FollowState {
    #[default]
    Normal,
    Tail {
        is_following: bool,
    },
}

impl FollowState {
    fn is_following(&self) -> bool {
        matches!(self, FollowState::Tail { is_following: true })
    }

    fn has_stopped_following(&self) -> bool {
        matches!(
            self,
            FollowState::Tail {
                is_following: false
            }
        )
    }

    fn start_following(&mut self) {
        if let FollowState::Tail {
            is_following: false,
        } = self
        {
            *self = FollowState::Tail { is_following: true };
        }
    }

    fn stop_following(&mut self) {
        if let FollowState::Tail { is_following: true } = self {
            *self = FollowState::Tail {
                is_following: false,
            };
        }
    }
}

/// Whether the list is scrolling from top to bottom or bottom to top.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum ListAlignment {
    /// The list is scrolling from top to bottom, like most lists.
    Top,
    /// The list is scrolling from bottom to top, like a chat log.
    Bottom,
}

/// A scroll event that has been converted to be in terms of the list's items.
pub struct ListScrollEvent {
    /// The range of items currently visible in the list, after applying the scroll event.
    pub visible_range: Range<usize>,

    /// The number of items that are currently visible in the list, after applying the scroll event.
    pub count: usize,

    /// Whether the list has been scrolled.
    pub is_scrolled: bool,

    /// Whether the list is currently in follow-tail mode.
    pub is_following_tail: bool,
}

/// The sizing behavior to apply during layout.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum ListSizingBehavior {
    /// The list should calculate its size based on the size of its items.
    Infer,
    /// The list should not calculate a fixed size.
    #[default]
    Auto,
}

/// The measuring behavior to apply during layout.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum ListMeasuringBehavior {
    /// Measure all items in the list.
    Measure(bool),
    /// Only measure visible items.
    #[default]
    Visible,
}

impl ListMeasuringBehavior {
    fn reset(&mut self) {
        match self {
            ListMeasuringBehavior::Measure(has_measured) => *has_measured = false,
            ListMeasuringBehavior::Visible => {}
        }
    }
}

/// The horizontal sizing behavior to apply during layout.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum ListHorizontalSizingBehavior {
    /// List items' width can never exceed the width of the list.
    #[default]
    FitList,
    /// List items' width may go over the width of the list, if any item is wider.
    Unconstrained,
}

struct LayoutItemsResponse {
    max_item_width: Pixels,
    scroll_top: ListOffset,
    item_layouts: VecDeque<ItemLayout>,
}

struct ItemLayout {
    index: usize,
    element: AnyElement,
    size: Size<Pixels>,
}

/// Frame state used by the [List] element after layout.
pub struct ListPrepaintState {
    hitbox: Hitbox,
    layout: LayoutItemsResponse,
}

#[derive(Clone)]
enum ListItem {
    Unmeasured {
        size_hint: Option<Size<Pixels>>,
        focus_handle: Option<FocusHandle>,
    },
    Measured {
        size: Size<Pixels>,
        focus_handle: Option<FocusHandle>,
    },
}

impl ListItem {
    fn size(&self) -> Option<Size<Pixels>> {
        if let ListItem::Measured { size, .. } = self {
            Some(*size)
        } else {
            None
        }
    }

    fn size_hint(&self) -> Option<Size<Pixels>> {
        match self {
            ListItem::Measured { size, .. } => Some(*size),
            ListItem::Unmeasured { size_hint, .. } => *size_hint,
        }
    }

    fn focus_handle(&self) -> Option<FocusHandle> {
        match self {
            ListItem::Unmeasured { focus_handle, .. } | ListItem::Measured { focus_handle, .. } => {
                focus_handle.clone()
            }
        }
    }

    fn contains_focused(&self, window: &Window, cx: &App) -> bool {
        match self {
            ListItem::Unmeasured { focus_handle, .. } | ListItem::Measured { focus_handle, .. } => {
                focus_handle
                    .as_ref()
                    .is_some_and(|handle| handle.contains_focused(window, cx))
            }
        }
    }
}

#[derive(Clone, Debug, Default, PartialEq)]
struct ListItemSummary {
    count: usize,
    rendered_count: usize,
    unrendered_count: usize,
    height: Pixels,
    has_focus_handles: bool,
    has_unknown_height: bool,
}

#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
struct Count(usize);

#[derive(Clone, Debug, Default)]
struct Height(Pixels);

impl ListState {
    /// Construct a new list state, for storage on a view.
    ///
    /// The overdraw parameter controls how much extra space is rendered
    /// above and below the visible area. Elements within this area will
    /// be measured even though they are not visible. This can help ensure
    /// that the list doesn't flicker or pop in when scrolling.
    pub fn new(item_count: usize, alignment: ListAlignment, overdraw: Pixels) -> Self {
        let this = Self(Rc::new(RefCell::new(StateInner {
            last_layout_bounds: None,
            last_padding: None,
            items: SumTree::default(),
            logical_scroll_top: None,
            alignment,
            overdraw,
            scroll_handler: None,
            reset: false,
            scrollbar_drag_start_height: None,
            measuring_behavior: ListMeasuringBehavior::default(),
            pending_scroll: None,
            follow_state: FollowState::default(),
        })));
        this.splice(0..0, item_count);
        this
    }

    /// Set the list to measure all items in the list in the first layout phase.
    ///
    /// This is useful for ensuring that the scrollbar size is correct instead of based on only rendered elements.
    pub fn measure_all(self) -> Self {
        self.0.borrow_mut().measuring_behavior = ListMeasuringBehavior::Measure(false);
        self
    }

    /// Pre-populate every unmeasured item with a uniform height hint so the scrollbar thumb
    /// is correctly sized from the first frame, without measuring all items up front.
    ///
    /// As items are actually rendered their real heights replace the hint, so the scrollbar
    /// converges to the exact size over time. This is a cheaper alternative to [`Self::measure_all`]
    /// for lists where items have roughly uniform heights (e.g. table rows).
    pub fn with_uniform_item_height(self, height: Pixels) -> Self {
        self.apply_uniform_item_height(height);
        self
    }

    /// Reset this instantiation of the list state.
    ///
    /// Note that this will cause scroll events to be dropped until the next paint.
    pub fn reset(&self, element_count: usize) {
        let old_count = {
            let state = &mut *self.0.borrow_mut();
            state.reset = true;
            state.measuring_behavior.reset();
            state.logical_scroll_top = None;
            state.pending_scroll = None;
            state.scrollbar_drag_start_height = None;
            state.items.summary().count
        };

        self.splice(0..old_count, element_count);
    }

    /// Reset the list to `element_count` items, pre-populating every item with a
    /// uniform height hint so the scrollbar thumb is correctly sized from the first
    /// frame even for off-screen items.
    pub fn reset_with_uniform_height(&self, element_count: usize, height: Pixels) {
        self.reset(element_count);
        self.apply_uniform_item_height(height);
    }

    fn apply_uniform_item_height(&self, height: Pixels) {
        let size_hint = Size {
            width: px(0.),
            height,
        };
        let mut state = self.0.borrow_mut();
        let new_items = state
            .items
            .iter()
            .map(|item| ListItem::Unmeasured {
                size_hint: Some(item.size_hint().unwrap_or(size_hint)),
                focus_handle: item.focus_handle(),
            })
            .collect::<Vec<_>>();
        let mut tree = SumTree::default();
        tree.extend(new_items, ());
        state.items = tree;
    }

    /// Remeasure all items while preserving proportional scroll position.
    pub fn remeasure(&self) {
        let count = self.item_count();
        self.remeasure_items_with_scroll_anchor(0..count, ScrollAnchor::Proportional);
    }

    /// Mark items in `range` as needing remeasurement while preserving
    /// the current scroll position.
    pub fn remeasure_items(&self, range: Range<usize>) {
        self.remeasure_items_with_scroll_anchor(range, ScrollAnchor::Absolute);
    }

    fn remeasure_items_with_scroll_anchor(&self, range: Range<usize>, scroll_anchor: ScrollAnchor) {
        let state = &mut *self.0.borrow_mut();

        if let Some(scroll_top) = state.logical_scroll_top
            && range.contains(&scroll_top.item_ix)
        {
            state.pending_scroll = match scroll_anchor {
                ScrollAnchor::Absolute => Some(PendingScroll::Absolute {
                    item_ix: scroll_top.item_ix,
                    offset: scroll_top.offset_in_item,
                }),
                ScrollAnchor::Proportional => {
                    let mut cursor = state.items.cursor::<Count>(());
                    cursor.seek(&Count(scroll_top.item_ix), Bias::Right);

                    cursor
                        .item()
                        .and_then(|item| {
                            item.size().map(|size| {
                                let fraction = if size.height > px(0.) {
                                    (scroll_top.offset_in_item / size.height).clamp(0.0, 1.0)
                                } else {
                                    0.0
                                };

                                PendingScroll::Proportional(PendingScrollFraction {
                                    item_ix: scroll_top.item_ix,
                                    fraction,
                                })
                            })
                        })
                        .or_else(|| state.pending_scroll.clone())
                }
            };
        }

        let new_items = {
            let mut cursor = state.items.cursor::<Count>(());
            let mut new_items = cursor.slice(&Count(range.start), Bias::Right);
            let invalidated = cursor.slice(&Count(range.end), Bias::Right);
            new_items.extend(
                invalidated.iter().map(|item| ListItem::Unmeasured {
                    size_hint: item.size_hint(),
                    focus_handle: item.focus_handle(),
                }),
                (),
            );
            new_items.append(cursor.suffix(), ());
            new_items
        };
        state.items = new_items;
        state.measuring_behavior.reset();
    }

    /// The number of items in this list.
    pub fn item_count(&self) -> usize {
        self.0.borrow().items.summary().count
    }

    /// Whether the list is scrolled to the end, or `None` if the list is
    /// not scrollable or the total content height is not yet known.
    pub fn is_scrolled_to_end(&self) -> Option<bool> {
        let state = self.0.borrow();
        let bounds = state.last_layout_bounds?;
        let summary = state.items.summary();
        if summary.has_unknown_height {
            return None;
        }
        let padding = state.last_padding.unwrap_or_default();
        let content_height = summary.height + padding.top + padding.bottom;
        let scroll_max = (content_height - bounds.size.height).max(px(0.));
        if scroll_max <= px(0.) {
            return None;
        }
        let scroll_top = state.scroll_top(&state.logical_scroll_top());
        Some(scroll_top >= scroll_max)
    }

    /// Inform the list state that the items in `old_range` have been replaced
    /// by `count` new items that must be recalculated.
    pub fn splice(&self, old_range: Range<usize>, count: usize) {
        self.splice_focusable(old_range, (0..count).map(|_| None))
    }

    /// Register with the list state that the items in `old_range` have been replaced
    /// by new items. As opposed to [`Self::splice`], this method allows an iterator of optional focus handles
    /// to be supplied to properly integrate with items in the list that can be focused. If a focused item
    /// is scrolled out of view, the list will continue to render it to allow keyboard interaction.
    pub fn splice_focusable(
        &self,
        old_range: Range<usize>,
        focus_handles: impl IntoIterator<Item = Option<FocusHandle>>,
    ) {
        let state = &mut *self.0.borrow_mut();

        let mut old_items = state.items.cursor::<Count>(());
        let mut new_items = old_items.slice(&Count(old_range.start), Bias::Right);
        old_items.seek_forward(&Count(old_range.end), Bias::Right);

        let mut spliced_count = 0;
        new_items.extend(
            focus_handles.into_iter().map(|focus_handle| {
                spliced_count += 1;
                ListItem::Unmeasured {
                    size_hint: None,
                    focus_handle,
                }
            }),
            (),
        );
        new_items.append(old_items.suffix(), ());
        drop(old_items);
        state.items = new_items;

        if let Some(ListOffset {
            item_ix,
            offset_in_item,
        }) = state.logical_scroll_top.as_mut()
        {
            if old_range.contains(item_ix) {
                *item_ix = old_range.start;
                *offset_in_item = px(0.);
            } else if old_range.end <= *item_ix {
                *item_ix = *item_ix - (old_range.end - old_range.start) + spliced_count;
            }
        }
    }

    /// Set a handler that will be called when the list is scrolled.
    pub fn set_scroll_handler(
        &self,
        handler: impl FnMut(&ListScrollEvent, &mut Window, &mut App) + 'static,
    ) {
        self.0.borrow_mut().scroll_handler = Some(Box::new(handler))
    }

    /// Get the current scroll offset, in terms of the list's items.
    pub fn logical_scroll_top(&self) -> ListOffset {
        self.0.borrow().logical_scroll_top()
    }

    /// Scroll the list by the given offset
    pub fn scroll_by(&self, distance: Pixels) {
        if distance == px(0.) {
            return;
        }

        let current_offset = self.logical_scroll_top();
        let state = &mut *self.0.borrow_mut();

        if distance < px(0.) {
            state.follow_state.stop_following();
        }

        let mut cursor = state.items.cursor::<ListItemSummary>(());
        cursor.seek(&Count(current_offset.item_ix), Bias::Right);

        let start_pixel_offset = cursor.start().height + current_offset.offset_in_item;
        let new_pixel_offset = (start_pixel_offset + distance).max(px(0.));
        if new_pixel_offset > start_pixel_offset {
            cursor.seek_forward(&Height(new_pixel_offset), Bias::Right);
        } else {
            cursor.seek(&Height(new_pixel_offset), Bias::Right);
        }

        let scroll_top = ListOffset {
            item_ix: cursor.start().count,
            offset_in_item: new_pixel_offset - cursor.start().height,
        };
        drop(cursor);
        state.rebase_pending_scroll(scroll_top);
        state.logical_scroll_top = Some(scroll_top);
    }

    /// Scroll the list to the very end, past the last item.
    pub fn scroll_to_end(&self) {
        let state = &mut *self.0.borrow_mut();
        let item_count = state.items.summary().count;
        state.pending_scroll = None;
        state.logical_scroll_top = Some(ListOffset {
            item_ix: item_count,
            offset_in_item: px(0.),
        });
    }

    /// Set the follow mode for the list.
    pub fn set_follow_mode(&self, mode: FollowMode) {
        let state = &mut *self.0.borrow_mut();

        match mode {
            FollowMode::Normal => {
                state.follow_state = FollowState::Normal;
            }
            FollowMode::Tail => {
                state.follow_state = FollowState::Tail { is_following: true };
                let item_count = state.items.summary().count;
                state.pending_scroll = None;
                state.logical_scroll_top = Some(ListOffset {
                    item_ix: item_count,
                    offset_in_item: px(0.),
                });
            }
        }
    }

    /// Compatibility helper for toggling follow-tail mode.
    pub fn set_follow_tail(&self, follow: bool) {
        self.set_follow_mode(if follow {
            FollowMode::Tail
        } else {
            FollowMode::Normal
        });
    }

    /// Returns whether the list is actively following the tail.
    pub fn is_following_tail(&self) -> bool {
        self.0.borrow().follow_state.is_following()
    }

    /// Scroll the list to the given offset
    pub fn scroll_to(&self, mut scroll_top: ListOffset) {
        let state = &mut *self.0.borrow_mut();
        let item_count = state.items.summary().count;
        if scroll_top.item_ix >= item_count {
            scroll_top.item_ix = item_count;
            scroll_top.offset_in_item = px(0.);
        }

        if scroll_top.item_ix < item_count {
            state.follow_state.stop_following();
        }

        state.rebase_pending_scroll(scroll_top);
        state.logical_scroll_top = Some(scroll_top);
    }

    /// Scroll the list to the given item, such that the item is fully visible.
    pub fn scroll_to_reveal_item(&self, ix: usize) {
        let state = &mut *self.0.borrow_mut();

        let mut scroll_top = state.logical_scroll_top();
        let height = state
            .last_layout_bounds
            .map_or(px(0.), |bounds| bounds.size.height);
        let padding = state.last_padding.unwrap_or_default();

        if ix <= scroll_top.item_ix {
            scroll_top.item_ix = ix;
            scroll_top.offset_in_item = px(0.);
        } else {
            let mut cursor = state.items.cursor::<ListItemSummary>(());
            cursor.seek(&Count(ix + 1), Bias::Right);
            let bottom = cursor.start().height + padding.top;
            let goal_top = px(0.).max(bottom - height + padding.bottom);

            cursor.seek(&Height(goal_top), Bias::Left);
            let start_ix = cursor.start().count;
            let start_item_top = cursor.start().height;

            if start_ix >= scroll_top.item_ix {
                scroll_top.item_ix = start_ix;
                scroll_top.offset_in_item = goal_top - start_item_top;
            }
        }

        state.rebase_pending_scroll(scroll_top);
        state.logical_scroll_top = Some(scroll_top);
    }

    /// Get the bounds for the given item in window coordinates, if it's
    /// been rendered.
    pub fn bounds_for_item(&self, ix: usize) -> Option<Bounds<Pixels>> {
        let state = &*self.0.borrow();

        let bounds = state.last_layout_bounds.unwrap_or_default();
        let scroll_top = state.logical_scroll_top();
        if ix < scroll_top.item_ix {
            return None;
        }

        let mut cursor = state.items.cursor::<Dimensions<Count, Height>>(());
        cursor.seek(&Count(scroll_top.item_ix), Bias::Right);

        let scroll_top = cursor.start().1.0 + scroll_top.offset_in_item;

        cursor.seek_forward(&Count(ix), Bias::Right);
        if let Some(&ListItem::Measured { size, .. }) = cursor.item() {
            let &Dimensions(Count(count), Height(top), _) = cursor.start();
            if count == ix {
                let top = bounds.top() + top - scroll_top;
                return Some(Bounds::from_corners(
                    point(bounds.left(), top),
                    point(bounds.right(), top + size.height),
                ));
            }
        }
        None
    }

    /// Call this method when the user starts dragging the scrollbar.
    ///
    /// This will prevent the height reported to the scrollbar from changing during the drag
    /// as items in the overdraw get measured, and help offset scroll position changes accordingly.
    pub fn scrollbar_drag_started(&self) {
        let mut state = self.0.borrow_mut();
        state.scrollbar_drag_start_height = Some(state.items.summary().height);
    }

    /// Called when the user stops dragging the scrollbar.
    ///
    /// See `scrollbar_drag_started`.
    pub fn scrollbar_drag_ended(&self) {
        self.0.borrow_mut().scrollbar_drag_start_height.take();
    }

    /// Returns `true` if the scrollbar is currently being dragged.
    pub fn is_scrollbar_dragging(&self) -> bool {
        self.0.borrow().scrollbar_drag_start_height.is_some()
    }

    /// Set the offset from the scrollbar
    pub fn set_offset_from_scrollbar(&self, point: Point<Pixels>) {
        self.0.borrow_mut().set_offset_from_scrollbar(point);
    }

    /// Returns the maximum scroll offset according to the items we have measured.
    /// This value remains constant while dragging to prevent the scrollbar from moving away unexpectedly.
    pub fn max_offset_for_scrollbar(&self) -> Size<Pixels> {
        let state = self.0.borrow();
        Size::new(Pixels::ZERO, state.max_scroll_offset())
    }

    /// Returns the current scroll offset adjusted for the scrollbar.
    ///
    /// The returned offset has a negative `y` component representing how far the content has
    /// scrolled.
    pub fn scroll_px_offset_for_scrollbar(&self) -> Point<Pixels> {
        let state = &self.0.borrow();

        if state.logical_scroll_top.is_none() && state.alignment == ListAlignment::Bottom {
            return Point::new(px(0.), -state.max_scroll_offset());
        }

        let logical_scroll_top = state.logical_scroll_top();

        let mut cursor = state.items.cursor::<ListItemSummary>(());
        let summary: ListItemSummary =
            cursor.summary(&Count(logical_scroll_top.item_ix), Bias::Right);
        let offset = summary.height + logical_scroll_top.offset_in_item;

        Point::new(px(0.), -offset)
    }

    /// Return the bounds of the viewport in pixels.
    pub fn viewport_bounds(&self) -> Bounds<Pixels> {
        self.0.borrow().last_layout_bounds.unwrap_or_default()
    }

    /// Returns whether the item is entirely above the viewport, or `None` if
    /// the list has not measured enough layout to know.
    pub fn item_is_above_viewport(&self, ix: usize) -> Option<bool> {
        let viewport_bounds = self.viewport_bounds();
        if viewport_bounds.size.height == px(0.0) {
            return None;
        }

        let scroll_top = self.logical_scroll_top();
        if ix < scroll_top.item_ix {
            return Some(true);
        }

        let item_bounds = self.bounds_for_item(ix)?;
        Some(item_bounds.bottom() <= viewport_bounds.top())
    }

    /// Returns whether the item is entirely below the viewport, or `None` if
    /// the list has not measured enough layout to know.
    pub fn item_is_below_viewport(&self, ix: usize) -> Option<bool> {
        let viewport_bounds = self.viewport_bounds();
        if viewport_bounds.size.height == px(0.0) {
            return None;
        }

        let scroll_top = self.logical_scroll_top();
        if ix < scroll_top.item_ix {
            return Some(false);
        }

        let item_bounds = self.bounds_for_item(ix)?;
        Some(item_bounds.top() >= viewport_bounds.bottom())
    }
}

impl StateInner {
    /// Re-anchor a pending scroll adjustment from a remeasure onto a newly set
    /// scroll position, so it clamps to the remeasured item's new height on
    /// the next layout instead of reverting the scroll.
    fn rebase_pending_scroll(&mut self, scroll_top: ListOffset) {
        let Some(pending) = self.pending_scroll.take() else {
            return;
        };
        if scroll_top.item_ix >= self.items.summary().count {
            return;
        }

        self.pending_scroll = match pending {
            PendingScroll::Absolute { .. } => Some(PendingScroll::Absolute {
                item_ix: scroll_top.item_ix,
                offset: scroll_top.offset_in_item,
            }),
            PendingScroll::Proportional(_) => {
                let mut cursor = self.items.cursor::<Count>(());
                cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
                cursor
                    .item()
                    .and_then(|item| item.size_hint())
                    .filter(|size| size.height > px(0.))
                    .map(|size| {
                        PendingScroll::Proportional(PendingScrollFraction {
                            item_ix: scroll_top.item_ix,
                            fraction: (scroll_top.offset_in_item / size.height).clamp(0.0, 1.0),
                        })
                    })
            }
        };
    }

    fn max_scroll_offset(&self) -> Pixels {
        let bounds = self.last_layout_bounds.unwrap_or_default();
        let height = self
            .scrollbar_drag_start_height
            .unwrap_or_else(|| self.items.summary().height);
        (height - bounds.size.height).max(px(0.))
    }

    fn visible_range(
        items: &SumTree<ListItem>,
        height: Pixels,
        scroll_top: &ListOffset,
    ) -> Range<usize> {
        let mut cursor = items.cursor::<ListItemSummary>(());
        cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
        let start_y = cursor.start().height + scroll_top.offset_in_item;
        cursor.seek_forward(&Height(start_y + height), Bias::Left);
        scroll_top.item_ix..cursor.start().count + 1
    }

    fn scroll(
        &mut self,
        scroll_top: &ListOffset,
        height: Pixels,
        delta: Point<Pixels>,
        current_view: EntityId,
        window: &mut Window,
        cx: &mut App,
    ) {
        // Drop scroll events after a reset, since we can't calculate
        // the new logical scroll top without the item heights
        if self.reset {
            return;
        }

        let padding = self.last_padding.unwrap_or_default();
        let scroll_max =
            (self.items.summary().height + padding.top + padding.bottom - height).max(px(0.));
        let new_scroll_top = (self.scroll_top(scroll_top) - delta.y)
            .max(px(0.))
            .min(scroll_max);

        let mut cursor = self.items.cursor::<ListItemSummary>(());
        cursor.seek(&Height(new_scroll_top), Bias::Right);
        let scroll_top_after_event = ListOffset {
            item_ix: cursor.start().count,
            offset_in_item: new_scroll_top - cursor.start().height,
        };
        drop(cursor);

        if self.alignment == ListAlignment::Bottom && new_scroll_top == scroll_max {
            self.pending_scroll = None;
            self.logical_scroll_top = None;
        } else {
            self.rebase_pending_scroll(scroll_top_after_event);
            self.logical_scroll_top = Some(scroll_top_after_event);
        }

        if delta.y > px(0.) {
            self.follow_state.stop_following();
        }

        if let Some(handler) = self.scroll_handler.as_mut() {
            let visible_range = Self::visible_range(&self.items, height, &scroll_top_after_event);
            let event = ListScrollEvent {
                visible_range,
                count: self.items.summary().count,
                is_scrolled: self.logical_scroll_top.is_some(),
                is_following_tail: self.follow_state.is_following(),
            };
            handler(&event, window, cx);
        }

        cx.notify(current_view);
    }

    fn logical_scroll_top(&self) -> ListOffset {
        self.logical_scroll_top
            .unwrap_or_else(|| match self.alignment {
                ListAlignment::Top => ListOffset {
                    item_ix: 0,
                    offset_in_item: px(0.),
                },
                ListAlignment::Bottom => ListOffset {
                    item_ix: self.items.summary().count,
                    offset_in_item: px(0.),
                },
            })
    }

    fn scroll_top(&self, logical_scroll_top: &ListOffset) -> Pixels {
        let mut cursor = self.items.cursor::<ListItemSummary>(());
        cursor.seek(&Count(logical_scroll_top.item_ix), Bias::Right);
        cursor.start().height + logical_scroll_top.offset_in_item
    }

    fn layout_all_items(
        &mut self,
        available_width: Pixels,
        render_item: &mut RenderItemFn,
        window: &mut Window,
        cx: &mut App,
    ) {
        match &mut self.measuring_behavior {
            ListMeasuringBehavior::Visible => return,
            ListMeasuringBehavior::Measure(has_measured) => {
                if *has_measured {
                    return;
                }
                *has_measured = true;
            }
        }

        let available_item_space = size(
            AvailableSpace::Definite(available_width),
            AvailableSpace::MinContent,
        );

        let measured_items = self.items.iter().enumerate().map(|(ix, item)| {
            let size = item.size().unwrap_or_else(|| {
                let mut element = render_item(ix, window, cx);
                element.layout_as_root(available_item_space, window, cx)
            });

            ListItem::Measured {
                size,
                focus_handle: item.focus_handle(),
            }
        });

        self.items = SumTree::from_iter(measured_items, ());
    }

    fn layout_items(
        &mut self,
        available_width: Option<Pixels>,
        available_height: Pixels,
        padding: &Edges<Pixels>,
        render_item: &mut RenderItemFn,
        window: &mut Window,
        cx: &mut App,
    ) -> LayoutItemsResponse {
        let old_items = self.items.clone();
        let mut measured_items = VecDeque::new();
        let mut item_layouts = VecDeque::new();
        let mut rendered_height = padding.top;
        let mut max_item_width = px(0.);
        let mut scroll_top = self.logical_scroll_top();

        if self.follow_state.is_following() {
            scroll_top = ListOffset {
                item_ix: self.items.summary().count,
                offset_in_item: px(0.),
            };
            self.logical_scroll_top = Some(scroll_top);
        }

        let mut rendered_focused_item = false;

        let available_item_space = size(
            crate::taffy::available_space_for_optional_width(available_width),
            AvailableSpace::MinContent,
        );

        let mut cursor = old_items.cursor::<Count>(());

        // Render items after the scroll top, including those in the trailing overdraw
        cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
        for (ix, item) in cursor.by_ref().enumerate() {
            let visible_height = rendered_height - scroll_top.offset_in_item;
            if visible_height >= available_height + self.overdraw {
                break;
            }

            // Use the previously cached height and focus handle if available
            let mut size = item.size();

            // If we're within the visible area or the height wasn't cached, render and measure the item's element
            if visible_height < available_height || size.is_none() {
                let item_index = scroll_top.item_ix + ix;
                let mut element = render_item(item_index, window, cx);
                let element_size = element.layout_as_root(available_item_space, window, cx);
                size = Some(element_size);

                if ix == 0
                    && let Some(pending_scroll) = self.pending_scroll.take()
                {
                    match pending_scroll {
                        PendingScroll::Absolute { item_ix, offset }
                            if item_ix == scroll_top.item_ix =>
                        {
                            scroll_top.offset_in_item = offset.min(element_size.height);
                            self.logical_scroll_top = Some(scroll_top);
                        }
                        PendingScroll::Proportional(pending_scroll)
                            if pending_scroll.item_ix == scroll_top.item_ix =>
                        {
                            scroll_top.offset_in_item =
                                element_size.height * pending_scroll.fraction;
                            self.logical_scroll_top = Some(scroll_top);
                        }
                        _ => {}
                    }
                }

                if visible_height < available_height {
                    item_layouts.push_back(ItemLayout {
                        index: item_index,
                        element,
                        size: element_size,
                    });
                    if item.contains_focused(window, cx) {
                        rendered_focused_item = true;
                    }
                }
            }

            let size = size.unwrap();
            rendered_height += size.height;
            max_item_width = max_item_width.max(size.width);
            measured_items.push_back(ListItem::Measured {
                size,
                focus_handle: item.focus_handle(),
            });
        }
        rendered_height += padding.bottom;

        // Prepare to start walking upward from the item at the scroll top.
        cursor.seek(&Count(scroll_top.item_ix), Bias::Right);

        // If the rendered items do not fill the visible region, then adjust
        // the scroll top upward.
        if rendered_height - scroll_top.offset_in_item < available_height {
            while rendered_height < available_height {
                cursor.prev();
                if let Some(item) = cursor.item() {
                    let item_index = cursor.start().0;
                    let mut element = render_item(item_index, window, cx);
                    let element_size = element.layout_as_root(available_item_space, window, cx);
                    let focus_handle = item.focus_handle();
                    rendered_height += element_size.height;
                    measured_items.push_front(ListItem::Measured {
                        size: element_size,
                        focus_handle,
                    });
                    item_layouts.push_front(ItemLayout {
                        index: item_index,
                        element,
                        size: element_size,
                    });
                    if item.contains_focused(window, cx) {
                        rendered_focused_item = true;
                    }
                } else {
                    break;
                }
            }

            scroll_top = ListOffset {
                item_ix: cursor.start().0,
                offset_in_item: rendered_height - available_height,
            };

            match self.alignment {
                ListAlignment::Top => {
                    scroll_top.offset_in_item = scroll_top.offset_in_item.max(px(0.));
                    self.logical_scroll_top = Some(scroll_top);
                }
                ListAlignment::Bottom => {
                    scroll_top = ListOffset {
                        item_ix: cursor.start().0,
                        offset_in_item: rendered_height - available_height,
                    };
                    self.logical_scroll_top = None;
                }
            };
        }

        // Measure items in the leading overdraw
        let mut leading_overdraw = scroll_top.offset_in_item;
        while leading_overdraw < self.overdraw {
            cursor.prev();
            if let Some(item) = cursor.item() {
                let size = if let ListItem::Measured { size, .. } = item {
                    *size
                } else {
                    let mut element = render_item(cursor.start().0, window, cx);
                    element.layout_as_root(available_item_space, window, cx)
                };

                leading_overdraw += size.height;
                measured_items.push_front(ListItem::Measured {
                    size,
                    focus_handle: item.focus_handle(),
                });
            } else {
                break;
            }
        }

        let measured_range = cursor.start().0..(cursor.start().0 + measured_items.len());
        let mut cursor = old_items.cursor::<Count>(());
        let mut new_items = cursor.slice(&Count(measured_range.start), Bias::Right);
        new_items.extend(measured_items, ());
        cursor.seek(&Count(measured_range.end), Bias::Right);
        new_items.append(cursor.suffix(), ());
        self.items = new_items;

        if self.follow_state.has_stopped_following() {
            let padding = self.last_padding.unwrap_or_default();
            let total_height = self.items.summary().height + padding.top + padding.bottom;
            let scroll_offset = self.scroll_top(&scroll_top);
            if scroll_offset + available_height >= total_height - px(1.0) {
                self.follow_state.start_following();
            }
        }

        // If none of the visible items are focused, check if an off-screen item is focused
        // and include it to be rendered after the visible items so keyboard interaction continues
        // to work for it.
        if !rendered_focused_item {
            let mut cursor = self
                .items
                .filter::<_, Count>((), |summary| summary.has_focus_handles);
            cursor.next();
            while let Some(item) = cursor.item() {
                if item.contains_focused(window, cx) {
                    let item_index = cursor.start().0;
                    let mut element = render_item(cursor.start().0, window, cx);
                    let size = element.layout_as_root(available_item_space, window, cx);
                    item_layouts.push_back(ItemLayout {
                        index: item_index,
                        element,
                        size,
                    });
                    break;
                }
                cursor.next();
            }
        }

        LayoutItemsResponse {
            max_item_width,
            scroll_top,
            item_layouts,
        }
    }

    fn prepaint_items(
        &mut self,
        bounds: Bounds<Pixels>,
        padding: Edges<Pixels>,
        autoscroll: bool,
        render_item: &mut RenderItemFn,
        window: &mut Window,
        cx: &mut App,
    ) -> Result<LayoutItemsResponse, ListOffset> {
        window.transact(|window| {
            if let ListMeasuringBehavior::Measure(false) = self.measuring_behavior {
                self.layout_all_items(bounds.size.width, render_item, window, cx);
            }

            let mut layout_response = self.layout_items(
                Some(bounds.size.width),
                bounds.size.height,
                &padding,
                render_item,
                window,
                cx,
            );

            // Avoid honoring autoscroll requests from elements other than our children.
            window.take_autoscroll();

            // Only paint the visible items, if there is actually any space for them (taking padding into account)
            if bounds.size.height > padding.top + padding.bottom {
                let mut item_origin = bounds.origin + Point::new(px(0.), padding.top);
                item_origin.y -= layout_response.scroll_top.offset_in_item;
                for item in &mut layout_response.item_layouts {
                    window.with_content_mask(Some(ContentMask { bounds }), |window| {
                        item.element.prepaint_at(item_origin, window, cx);
                    });

                    if let Some(autoscroll_bounds) = window.take_autoscroll()
                        && autoscroll
                    {
                        if autoscroll_bounds.top() < bounds.top() {
                            return Err(ListOffset {
                                item_ix: item.index,
                                offset_in_item: autoscroll_bounds.top() - item_origin.y,
                            });
                        } else if autoscroll_bounds.bottom() > bounds.bottom() {
                            let mut cursor = self.items.cursor::<Count>(());
                            cursor.seek(&Count(item.index), Bias::Right);
                            let mut height = bounds.size.height - padding.top - padding.bottom;

                            // Account for the height of the element down until the autoscroll bottom.
                            height -= autoscroll_bounds.bottom() - item_origin.y;

                            // Keep decreasing the scroll top until we fill all the available space.
                            while height > Pixels::ZERO {
                                cursor.prev();
                                let Some(item) = cursor.item() else { break };

                                let size = item.size().unwrap_or_else(|| {
                                    let mut item = render_item(cursor.start().0, window, cx);
                                    let item_available_size =
                                        size(bounds.size.width.into(), AvailableSpace::MinContent);
                                    item.layout_as_root(item_available_size, window, cx)
                                });
                                height -= size.height;
                            }

                            return Err(ListOffset {
                                item_ix: cursor.start().0,
                                offset_in_item: if height < Pixels::ZERO {
                                    -height
                                } else {
                                    Pixels::ZERO
                                },
                            });
                        }
                    }

                    item_origin.y += item.size.height;
                }
            } else {
                layout_response.item_layouts.clear();
            }

            Ok(layout_response)
        })
    }

    // Scrollbar support

    fn set_offset_from_scrollbar(&mut self, point: Point<Pixels>) {
        let Some(bounds) = self.last_layout_bounds else {
            return;
        };
        let height = bounds.size.height;

        let padding = self.last_padding.unwrap_or_default();
        let content_height = self
            .scrollbar_drag_start_height
            .unwrap_or_else(|| self.items.summary().height);
        let scroll_max = (content_height + padding.top + padding.bottom - height).max(px(0.));
        let new_scroll_top = (-point.y).max(px(0.)).min(scroll_max);

        let dragged_to_end =
            scroll_max > px(0.) && new_scroll_top >= (scroll_max - px(1.)).max(px(0.));
        if dragged_to_end && matches!(self.follow_state, FollowState::Tail { .. }) {
            self.follow_state = FollowState::Tail { is_following: true };
            let item_count = self.items.summary().count;
            self.pending_scroll = None;
            self.logical_scroll_top = Some(ListOffset {
                item_ix: item_count,
                offset_in_item: px(0.),
            });
            return;
        }

        self.follow_state.stop_following();

        if self.alignment == ListAlignment::Bottom && new_scroll_top == scroll_max {
            self.pending_scroll = None;
            self.logical_scroll_top = None;
        } else {
            let mut cursor = self.items.cursor::<ListItemSummary>(());
            cursor.seek(&Height(new_scroll_top), Bias::Right);

            let scroll_top = ListOffset {
                item_ix: cursor.start().count,
                offset_in_item: new_scroll_top - cursor.start().height,
            };
            drop(cursor);
            self.rebase_pending_scroll(scroll_top);
            self.logical_scroll_top = Some(scroll_top);
        }
    }
}

impl std::fmt::Debug for ListItem {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Unmeasured { .. } => write!(f, "Unrendered"),
            Self::Measured { size, .. } => f.debug_struct("Rendered").field("size", size).finish(),
        }
    }
}

/// An offset into the list's items, in terms of the item index and the number
/// of pixels off the top left of the item.
#[derive(Debug, Clone, Copy, Default)]
pub struct ListOffset {
    /// The index of an item in the list
    pub item_ix: usize,
    /// The number of pixels to offset from the item index.
    pub offset_in_item: Pixels,
}

impl Element for List {
    type RequestLayoutState = ();
    type PrepaintState = ListPrepaintState;

    fn id(&self) -> Option<crate::ElementId> {
        None
    }

    fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
        None
    }

    fn request_layout(
        &mut self,
        _id: Option<&GlobalElementId>,
        _inspector_id: Option<&InspectorElementId>,
        window: &mut Window,
        cx: &mut App,
    ) -> (crate::LayoutId, Self::RequestLayoutState) {
        let layout_id = match self.sizing_behavior {
            ListSizingBehavior::Infer => {
                let mut style = Style::default();
                style.overflow.y = Overflow::Scroll;
                style.refine(&self.style);
                window.with_text_style(style.text_style().cloned(), |window| {
                    let state = &mut *self.state.0.borrow_mut();

                    let available_height = if let Some(last_bounds) = state.last_layout_bounds {
                        last_bounds.size.height
                    } else {
                        // If we don't have the last layout bounds (first render),
                        // we might just use the overdraw value as the available height to layout enough items.
                        state.overdraw
                    };
                    let padding = style.padding.to_pixels(
                        state.last_layout_bounds.unwrap_or_default().size.into(),
                        window.rem_size(),
                    );

                    let layout_response = state.layout_items(
                        None,
                        available_height,
                        &padding,
                        &mut self.render_item,
                        window,
                        cx,
                    );
                    let max_element_width = layout_response.max_item_width;

                    let summary = state.items.summary();
                    let total_height = summary.height;

                    window.request_measured_layout(
                        style,
                        move |known_dimensions, available_space, _window, _cx| {
                            let width =
                                known_dimensions
                                    .width
                                    .unwrap_or(match available_space.width {
                                        AvailableSpace::Definite(x) => x,
                                        AvailableSpace::MinContent | AvailableSpace::MaxContent => {
                                            max_element_width
                                        }
                                    });
                            let height = match available_space.height {
                                AvailableSpace::Definite(height) => total_height.min(height),
                                AvailableSpace::MinContent | AvailableSpace::MaxContent => {
                                    total_height
                                }
                            };
                            size(width, height)
                        },
                    )
                })
            }
            ListSizingBehavior::Auto => {
                let mut style = Style::default();
                style.refine(&self.style);
                window.with_text_style(style.text_style().cloned(), |window| {
                    window.request_layout(style, None, cx)
                })
            }
        };
        (layout_id, ())
    }

    fn prepaint(
        &mut self,
        _id: Option<&GlobalElementId>,
        _inspector_id: Option<&InspectorElementId>,
        bounds: Bounds<Pixels>,
        _: &mut Self::RequestLayoutState,
        window: &mut Window,
        cx: &mut App,
    ) -> ListPrepaintState {
        let state = &mut *self.state.0.borrow_mut();
        state.reset = false;

        let mut style = Style::default();
        style.refine(&self.style);

        let hitbox = window.insert_hitbox(bounds, HitboxBehavior::Normal);

        // If the width of the list has changed, invalidate all cached item heights
        if state
            .last_layout_bounds
            .is_none_or(|last_bounds| last_bounds.size.width != bounds.size.width)
        {
            let new_items = SumTree::from_iter(
                state.items.iter().map(|item| ListItem::Unmeasured {
                    size_hint: item.size_hint(),
                    focus_handle: item.focus_handle(),
                }),
                (),
            );

            state.items = new_items;
            state.measuring_behavior.reset();
        }

        let padding = style
            .padding
            .to_pixels(bounds.size.into(), window.rem_size());
        let layout =
            match state.prepaint_items(bounds, padding, true, &mut self.render_item, window, cx) {
                Ok(layout) => layout,
                Err(autoscroll_request) => {
                    state.logical_scroll_top = Some(autoscroll_request);
                    state
                        .prepaint_items(bounds, padding, false, &mut self.render_item, window, cx)
                        .unwrap()
                }
            };

        state.last_layout_bounds = Some(bounds);
        state.last_padding = Some(padding);
        ListPrepaintState { hitbox, layout }
    }

    fn paint(
        &mut self,
        _id: Option<&GlobalElementId>,
        _inspector_id: Option<&InspectorElementId>,
        bounds: Bounds<crate::Pixels>,
        _: &mut Self::RequestLayoutState,
        prepaint: &mut Self::PrepaintState,
        window: &mut Window,
        cx: &mut App,
    ) {
        let current_view = window.current_view();
        window.with_content_mask(Some(ContentMask { bounds }), |window| {
            for item in &mut prepaint.layout.item_layouts {
                item.element.paint(window, cx);
            }
        });

        let list_state = self.state.clone();
        let height = bounds.size.height;
        let scroll_top = prepaint.layout.scroll_top;
        let hitbox_id = prepaint.hitbox.id;
        let mut accumulated_scroll_delta = ScrollDelta::default();
        window.on_mouse_event(move |event: &ScrollWheelEvent, phase, window, cx| {
            if phase == DispatchPhase::Bubble && hitbox_id.should_handle_scroll(window) {
                accumulated_scroll_delta = accumulated_scroll_delta.coalesce(event.delta);
                let pixel_delta = accumulated_scroll_delta.pixel_delta(px(20.));
                list_state.0.borrow_mut().scroll(
                    &scroll_top,
                    height,
                    pixel_delta,
                    current_view,
                    window,
                    cx,
                )
            }
        });
    }
}

impl IntoElement for List {
    type Element = Self;

    fn into_element(self) -> Self::Element {
        self
    }
}

impl Styled for List {
    fn style(&mut self) -> &mut StyleRefinement {
        &mut self.style
    }
}

impl sum_tree::Item for ListItem {
    type Summary = ListItemSummary;

    fn summary(&self, _: ()) -> Self::Summary {
        match self {
            ListItem::Unmeasured {
                size_hint,
                focus_handle,
            } => ListItemSummary {
                count: 1,
                rendered_count: 0,
                unrendered_count: 1,
                height: size_hint.map_or(px(0.), |size| size.height),
                has_focus_handles: focus_handle.is_some(),
                has_unknown_height: size_hint.is_none(),
            },
            ListItem::Measured {
                size, focus_handle, ..
            } => ListItemSummary {
                count: 1,
                rendered_count: 1,
                unrendered_count: 0,
                height: size.height,
                has_focus_handles: focus_handle.is_some(),
                has_unknown_height: false,
            },
        }
    }
}

impl sum_tree::ContextLessSummary for ListItemSummary {
    fn zero() -> Self {
        Default::default()
    }

    fn add_summary(&mut self, summary: &Self) {
        self.count += summary.count;
        self.rendered_count += summary.rendered_count;
        self.unrendered_count += summary.unrendered_count;
        self.height += summary.height;
        self.has_focus_handles |= summary.has_focus_handles;
        self.has_unknown_height |= summary.has_unknown_height;
    }
}

impl<'a> sum_tree::Dimension<'a, ListItemSummary> for Count {
    fn zero(_cx: ()) -> Self {
        Default::default()
    }

    fn add_summary(&mut self, summary: &'a ListItemSummary, _: ()) {
        self.0 += summary.count;
    }
}

impl<'a> sum_tree::Dimension<'a, ListItemSummary> for Height {
    fn zero(_cx: ()) -> Self {
        Default::default()
    }

    fn add_summary(&mut self, summary: &'a ListItemSummary, _: ()) {
        self.0 += summary.height;
    }
}

impl sum_tree::SeekTarget<'_, ListItemSummary, ListItemSummary> for Count {
    fn cmp(&self, other: &ListItemSummary, _: ()) -> std::cmp::Ordering {
        self.0.partial_cmp(&other.count).unwrap()
    }
}

impl sum_tree::SeekTarget<'_, ListItemSummary, ListItemSummary> for Height {
    fn cmp(&self, other: &ListItemSummary, _: ()) -> std::cmp::Ordering {
        self.0.partial_cmp(&other.height).unwrap()
    }
}

#[cfg(test)]
mod test {

    use gpui::{ScrollDelta, ScrollWheelEvent};

    use crate::{
        self as gpui, AppContext, Context, Element, IntoElement, ListState, Render, Styled,
        TestAppContext, Window, div, list, point, px, size,
    };

    struct TestListView(ListState);

    impl Render for TestListView {
        fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
            list(self.0.clone(), |_, _, _| {
                div().h(px(20.)).w_full().into_any()
            })
            .w_full()
            .h_full()
        }
    }

    #[gpui::test]
    fn test_uniform_item_height_populates_size_hints(_cx: &mut TestAppContext) {
        let state =
            ListState::new(5, crate::ListAlignment::Top, px(0.)).with_uniform_item_height(px(20.));

        let summary = state.0.borrow().items.summary().clone();
        assert_eq!(summary.count, 5);
        assert_eq!(summary.rendered_count, 0);
        assert_eq!(summary.unrendered_count, 5);
        assert_eq!(summary.height, px(100.));
        assert_eq!(state.max_offset_for_scrollbar().height, px(100.));

        state.reset_with_uniform_height(3, px(10.));

        let summary = state.0.borrow().items.summary().clone();
        assert_eq!(summary.count, 3);
        assert_eq!(summary.rendered_count, 0);
        assert_eq!(summary.unrendered_count, 3);
        assert_eq!(summary.height, px(30.));
        assert_eq!(state.max_offset_for_scrollbar().height, px(30.));
    }

    #[gpui::test]
    fn test_is_scrolled_to_end_is_unknown_until_item_heights_are_known(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(0.));

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(40.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        assert_eq!(state.is_scrolled_to_end(), None);
    }

    #[gpui::test]
    fn test_is_scrolled_to_end_reports_when_item_heights_are_known(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(0.)).measure_all();

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(40.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        assert_eq!(state.is_scrolled_to_end(), Some(false));

        state.scroll_to_end();

        assert_eq!(state.is_scrolled_to_end(), Some(true));
    }

    #[gpui::test]
    fn test_reset_after_paint_before_scroll(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };

        let cx = cx.add_empty_window();

        let state = ListState::new(5, crate::ListAlignment::Top, px(10.));

        // Ensure that the list is scrolled to the top
        state.scroll_to(gpui::ListOffset {
            item_ix: 0,
            offset_in_item: px(0.0),
        });

        struct TestView(ListState);
        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                list(self.0.clone(), |_, _, _| {
                    div().h(px(10.)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        // Paint
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
            cx.new(|_| TestView(state.clone()))
        });

        // Reset
        state.reset(5);

        // And then receive a scroll event _before_ the next paint
        cx.simulate_event(ScrollWheelEvent {
            position: point(px(1.), px(1.)),
            delta: ScrollDelta::Pixels(point(px(0.), px(-500.))),
            ..Default::default()
        });

        // Scroll position should stay at the top of the list
        assert_eq!(state.logical_scroll_top().item_ix, 0);
        assert_eq!(state.logical_scroll_top().offset_in_item, px(0.));
    }

    #[gpui::test]
    fn test_scroll_by_positive_and_negative_distance(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };

        let cx = cx.add_empty_window();

        let state = ListState::new(5, crate::ListAlignment::Top, px(10.));

        struct TestView(ListState);
        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                list(self.0.clone(), |_, _, _| {
                    div().h(px(20.)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        // Paint
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(100.)), |_, cx| {
            cx.new(|_| TestView(state.clone()))
        });

        // Test positive distance: start at item 1, move down 30px
        state.scroll_by(px(30.));

        // Should move to item 2
        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 1);
        assert_eq!(offset.offset_in_item, px(10.));

        // Test negative distance: start at item 2, move up 30px
        state.scroll_by(px(-30.));

        // Should move back to item 1
        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 0);
        assert_eq!(offset.offset_in_item, px(0.));

        // Test zero distance
        state.scroll_by(px(0.));
        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 0);
        assert_eq!(offset.offset_in_item, px(0.));
    }

    #[gpui::test]
    fn test_item_viewport_queries_return_none_before_layout(_cx: &mut TestAppContext) {
        let state = ListState::new(5, crate::ListAlignment::Top, px(10.)).measure_all();

        assert_eq!(state.item_is_above_viewport(0), None);
        assert_eq!(state.item_is_below_viewport(0), None);
    }

    #[gpui::test]
    fn test_item_viewport_queries_before_logical_scroll_top(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(10.)).measure_all();

        state.scroll_to(gpui::ListOffset {
            item_ix: 2,
            offset_in_item: px(0.),
        });
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        assert_eq!(state.item_is_above_viewport(1), Some(true));
        assert_eq!(state.item_is_below_viewport(1), Some(false));
    }

    #[gpui::test]
    fn test_item_viewport_queries_measured_item_inside_viewport(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(10.)).measure_all();

        state.scroll_to(gpui::ListOffset {
            item_ix: 2,
            offset_in_item: px(0.),
        });
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        assert_eq!(state.item_is_above_viewport(2), Some(false));
        assert_eq!(state.item_is_below_viewport(2), Some(false));
    }

    #[gpui::test]
    fn test_item_viewport_queries_measured_item_above_viewport(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(10.)).measure_all();

        state.scroll_to(gpui::ListOffset {
            item_ix: 2,
            offset_in_item: px(20.),
        });
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        assert_eq!(state.item_is_above_viewport(2), Some(true));
        assert_eq!(state.item_is_below_viewport(2), Some(false));
    }

    #[gpui::test]
    fn test_item_viewport_queries_measured_item_below_viewport(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(10.)).measure_all();

        state.scroll_to(gpui::ListOffset {
            item_ix: 2,
            offset_in_item: px(0.),
        });
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        assert_eq!(state.item_is_above_viewport(3), Some(false));
        assert_eq!(state.item_is_below_viewport(3), Some(true));
    }

    #[gpui::test]
    fn test_item_viewport_queries_after_scroll_to_end_before_layout(cx: &mut TestAppContext) {
        let cx = cx.add_empty_window();
        let state = ListState::new(5, crate::ListAlignment::Top, px(10.)).measure_all();

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
            cx.new(|_| TestListView(state.clone())).into_any_element()
        });

        state.scroll_to_end();

        assert_eq!(state.logical_scroll_top().item_ix, state.item_count());
        assert_eq!(state.item_is_above_viewport(0), Some(true));
        assert_eq!(state.item_is_below_viewport(0), Some(false));
    }

    #[gpui::test]
    fn test_scroll_handler_reports_visible_range_after_scroll(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };
        use std::{cell::RefCell, rc::Rc};

        let cx = cx.add_empty_window();

        let state = ListState::new(5, crate::ListAlignment::Top, px(0.)).measure_all();
        let visible_range = Rc::new(RefCell::new(None));
        let observed_visible_range = visible_range.clone();
        state.set_scroll_handler(move |event, _, _| {
            *observed_visible_range.borrow_mut() = Some(event.visible_range.clone());
        });

        struct TestView(ListState);
        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                list(self.0.clone(), |_, _, _| {
                    div().h(px(20.)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(40.)), |_, cx| {
            cx.new(|_| TestView(state.clone()))
        });

        cx.simulate_event(ScrollWheelEvent {
            position: point(px(1.), px(1.)),
            delta: ScrollDelta::Pixels(point(px(0.), px(-20.))),
            ..Default::default()
        });

        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 1);
        assert_eq!(offset.offset_in_item, px(0.));

        let visible_range = visible_range.borrow().clone().unwrap();
        assert_eq!(visible_range.start, offset.item_ix);
    }

    #[gpui::test]
    fn test_remeasure_items_preserves_size_hints_for_scrollbar(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };

        let cx = cx.add_empty_window();

        let state = ListState::new(5, crate::ListAlignment::Top, px(0.)).measure_all();

        struct TestView(ListState);
        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                list(self.0.clone(), |_, _, _| {
                    div().h(px(20.)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(40.)), |_, cx| {
            cx.new(|_| TestView(state.clone()))
        });

        assert_eq!(state.max_offset_for_scrollbar().height, px(60.));

        state.remeasure_items(0..5);

        let summary = state.0.borrow().items.summary().clone();
        assert_eq!(summary.rendered_count, 0);
        assert_eq!(summary.unrendered_count, 5);
        assert_eq!(summary.height, px(100.));
        assert_eq!(state.max_offset_for_scrollbar().height, px(60.));
    }

    #[gpui::test]
    fn test_remeasure_item_preserves_scroll_offset(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };
        use std::{cell::Cell, rc::Rc};

        let cx = cx.add_empty_window();
        let item_height = Rc::new(Cell::new(100usize));
        let state = ListState::new(20, crate::ListAlignment::Top, px(10.));

        struct TestView {
            state: ListState,
            item_height: Rc<Cell<usize>>,
        }

        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                let height = self.item_height.get();
                list(self.state.clone(), move |index, _, _| {
                    let height = if index == 5 { height } else { 100 };
                    div().h(px(height as f32)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        let view = cx.update(|_, cx| {
            cx.new(|_| TestView {
                state: state.clone(),
                item_height: item_height.clone(),
            })
        });

        state.scroll_to(gpui::ListOffset {
            item_ix: 5,
            offset_in_item: px(40.),
        });

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });

        item_height.set(200);
        state.remeasure_items(5..6);

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.into_any_element()
        });

        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 5);
        assert_eq!(offset.offset_in_item, px(40.));
    }

    #[gpui::test]
    fn test_remeasure_then_scroll_does_not_revert_scroll_position(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };

        let cx = cx.add_empty_window();
        let state = ListState::new(20, crate::ListAlignment::Top, px(10.));

        struct TestView(ListState);
        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                list(self.0.clone(), |_, _, _| {
                    div().h(px(100.)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        let view = cx.update(|_, cx| cx.new(|_| TestView(state.clone())));

        state.scroll_to(gpui::ListOffset {
            item_ix: 5,
            offset_in_item: px(40.),
        });

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });

        state.remeasure_items(5..6);

        cx.simulate_event(ScrollWheelEvent {
            position: point(px(50.), px(100.)),
            delta: ScrollDelta::Pixels(point(px(0.), px(-30.))),
            ..Default::default()
        });

        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 5);
        assert_eq!(offset.offset_in_item, px(70.));

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.into_any_element()
        });

        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 5);
        assert_eq!(
            offset.offset_in_item,
            px(70.),
            "scrolling after a remeasure should not be reverted by the stale pending scroll"
        );
    }

    #[gpui::test]
    fn test_scroll_after_remeasure_clamps_to_shrunk_item_height(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };
        use std::{cell::Cell, rc::Rc};

        let cx = cx.add_empty_window();
        let item_height = Rc::new(Cell::new(100usize));
        let state = ListState::new(20, crate::ListAlignment::Top, px(10.));

        struct TestView {
            state: ListState,
            item_height: Rc<Cell<usize>>,
        }

        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                let height = self.item_height.get();
                list(self.state.clone(), move |index, _, _| {
                    let height = if index == 5 { height } else { 100 };
                    div().h(px(height as f32)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        let view = cx.update(|_, cx| {
            cx.new(|_| TestView {
                state: state.clone(),
                item_height: item_height.clone(),
            })
        });

        state.scroll_to(gpui::ListOffset {
            item_ix: 5,
            offset_in_item: px(40.),
        });

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });

        item_height.set(50);
        state.remeasure_items(5..6);

        cx.simulate_event(ScrollWheelEvent {
            position: point(px(50.), px(100.)),
            delta: ScrollDelta::Pixels(point(px(0.), px(-30.))),
            ..Default::default()
        });

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.into_any_element()
        });

        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 5);
        assert_eq!(offset.offset_in_item, px(50.));
    }

    #[gpui::test]
    fn test_scrollbar_drag_with_growing_content(cx: &mut TestAppContext) {
        use crate::{
            AppContext, Context, Element, IntoElement, ListState, Render, Styled, Window, div,
            list, point, px, size,
        };
        use std::{cell::Cell, rc::Rc};

        let cx = cx.add_empty_window();
        let last_item_height = Rc::new(Cell::new(50usize));
        let state = ListState::new(10, crate::ListAlignment::Top, px(0.)).measure_all();

        struct TestView {
            state: ListState,
            last_item_height: Rc<Cell<usize>>,
        }

        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                let last_item_height = self.last_item_height.clone();
                list(self.state.clone(), move |index, _, _| {
                    let height = if index == 9 {
                        last_item_height.get()
                    } else {
                        50
                    };
                    div().h(px(height as f32)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        let view = cx.update(|_, cx| {
            cx.new(|_| TestView {
                state: state.clone(),
                last_item_height: last_item_height.clone(),
            })
        });

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });

        state.scrollbar_drag_started();
        state.set_offset_from_scrollbar(point(px(0.), px(-150.)));
        let scrollbar_offset_before_growth = state.scroll_px_offset_for_scrollbar();

        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 3);
        assert_eq!(offset.offset_in_item, px(0.));

        last_item_height.set(550);
        state.remeasure_items(9..10);
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });

        assert_eq!(state.max_offset_for_scrollbar().height, px(300.));
        assert_eq!(
            state.scroll_px_offset_for_scrollbar(),
            scrollbar_offset_before_growth
        );

        state.set_offset_from_scrollbar(point(px(0.), px(-150.)));
        let offset = state.logical_scroll_top();
        assert_eq!(offset.item_ix, 3);
        assert_eq!(offset.offset_in_item, px(0.));
    }

    #[gpui::test]
    fn test_follow_tail_reengages_after_scrollbar_drag_to_bottom_while_growing(
        cx: &mut TestAppContext,
    ) {
        use crate::{
            AppContext, Context, Element, FollowMode, IntoElement, ListState, Render, Styled,
            Window, div, list, point, px, size,
        };

        let cx = cx.add_empty_window();
        let state = ListState::new(10, crate::ListAlignment::Top, px(0.)).measure_all();

        struct TestView(ListState);
        impl Render for TestView {
            fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
                list(self.0.clone(), |_, _, _| {
                    div().h(px(50.)).w_full().into_any()
                })
                .w_full()
                .h_full()
            }
        }

        let view = cx.update(|_, cx| cx.new(|_| TestView(state.clone())));

        state.set_follow_mode(FollowMode::Tail);
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });
        assert!(state.is_following_tail());

        state.scrollbar_drag_started();

        state.splice(10..10, 10);
        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.clone().into_any_element()
        });

        state.set_offset_from_scrollbar(point(px(0.), px(-300.)));
        state.scrollbar_drag_ended();

        cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
            view.into_any_element()
        });

        assert!(state.is_following_tail());
    }
}