Move all crates to a top-level crates folder

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/gpui/src/elements/list.rs

@@ -0,0 +1,890 @@
+use crate::{
+    geometry::{
+        rect::RectF,
+        vector::{vec2f, Vector2F},
+    },
+    json::json,
+    DebugContext, Element, ElementBox, ElementRc, Event, EventContext, LayoutContext, PaintContext,
+    SizeConstraint,
+};
+use std::{cell::RefCell, collections::VecDeque, ops::Range, rc::Rc};
+use sum_tree::{self, Bias, SumTree};
+
+pub struct List {
+    state: ListState,
+    invalidated_elements: Vec<ElementRc>,
+}
+
+#[derive(Clone)]
+pub struct ListState(Rc<RefCell<StateInner>>);
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum Orientation {
+    Top,
+    Bottom,
+}
+
+struct StateInner {
+    last_layout_width: Option<f32>,
+    render_item: Box<dyn FnMut(usize, &mut LayoutContext) -> ElementBox>,
+    rendered_range: Range<usize>,
+    items: SumTree<ListItem>,
+    logical_scroll_top: Option<ListOffset>,
+    orientation: Orientation,
+    overdraw: f32,
+    scroll_handler: Option<Box<dyn FnMut(Range<usize>, &mut EventContext)>>,
+}
+
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+pub struct ListOffset {
+    item_ix: usize,
+    offset_in_item: f32,
+}
+
+#[derive(Clone)]
+enum ListItem {
+    Unrendered,
+    Rendered(ElementRc),
+    Removed(f32),
+}
+
+impl std::fmt::Debug for ListItem {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Unrendered => write!(f, "Unrendered"),
+            Self::Rendered(_) => f.debug_tuple("Rendered").finish(),
+            Self::Removed(height) => f.debug_tuple("Removed").field(height).finish(),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Default, PartialEq)]
+struct ListItemSummary {
+    count: usize,
+    rendered_count: usize,
+    unrendered_count: usize,
+    height: f32,
+}
+
+#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+struct Count(usize);
+
+#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+struct RenderedCount(usize);
+
+#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+struct UnrenderedCount(usize);
+
+#[derive(Clone, Debug, Default)]
+struct Height(f32);
+
+impl List {
+    pub fn new(state: ListState) -> Self {
+        Self {
+            state,
+            invalidated_elements: Default::default(),
+        }
+    }
+}
+
+impl Element for List {
+    type LayoutState = ListOffset;
+    type PaintState = ();
+
+    fn layout(
+        &mut self,
+        constraint: SizeConstraint,
+        cx: &mut LayoutContext,
+    ) -> (Vector2F, Self::LayoutState) {
+        let state = &mut *self.state.0.borrow_mut();
+        let size = constraint.max;
+        let mut item_constraint = constraint;
+        item_constraint.min.set_y(0.);
+        item_constraint.max.set_y(f32::INFINITY);
+
+        if cx.refreshing || state.last_layout_width != Some(size.x()) {
+            state.rendered_range = 0..0;
+            state.items = SumTree::from_iter(
+                (0..state.items.summary().count).map(|_| ListItem::Unrendered),
+                &(),
+            )
+        }
+
+        let old_items = state.items.clone();
+        let mut new_items = SumTree::new();
+        let mut rendered_items = VecDeque::new();
+        let mut rendered_height = 0.;
+        let mut scroll_top = state
+            .logical_scroll_top
+            .unwrap_or_else(|| match state.orientation {
+                Orientation::Top => ListOffset {
+                    item_ix: 0,
+                    offset_in_item: 0.,
+                },
+                Orientation::Bottom => ListOffset {
+                    item_ix: state.items.summary().count,
+                    offset_in_item: 0.,
+                },
+            });
+
+        // Render items after the scroll top, including those in the trailing overdraw.
+        let mut cursor = old_items.cursor::<Count>();
+        cursor.seek(&Count(scroll_top.item_ix), Bias::Right, &());
+        for (ix, item) in cursor.by_ref().enumerate() {
+            if rendered_height - scroll_top.offset_in_item >= size.y() + state.overdraw {
+                break;
+            }
+
+            let element = state.render_item(scroll_top.item_ix + ix, item, item_constraint, cx);
+            rendered_height += element.size().y();
+            rendered_items.push_back(ListItem::Rendered(element));
+        }
+
+        // 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 < size.y() {
+            while rendered_height < size.y() {
+                cursor.prev(&());
+                if let Some(item) = cursor.item() {
+                    let element = state.render_item(cursor.start().0, item, item_constraint, cx);
+                    rendered_height += element.size().y();
+                    rendered_items.push_front(ListItem::Rendered(element));
+                } else {
+                    break;
+                }
+            }
+
+            scroll_top = ListOffset {
+                item_ix: cursor.start().0,
+                offset_in_item: rendered_height - size.y(),
+            };
+
+            match state.orientation {
+                Orientation::Top => {
+                    scroll_top.offset_in_item = scroll_top.offset_in_item.max(0.);
+                    state.logical_scroll_top = Some(scroll_top);
+                }
+                Orientation::Bottom => {
+                    scroll_top = ListOffset {
+                        item_ix: cursor.start().0,
+                        offset_in_item: rendered_height - size.y(),
+                    };
+                    state.logical_scroll_top = None;
+                }
+            };
+        }
+
+        // Render items in the leading overdraw.
+        let mut leading_overdraw = scroll_top.offset_in_item;
+        while leading_overdraw < state.overdraw {
+            cursor.prev(&());
+            if let Some(item) = cursor.item() {
+                let element = state.render_item(cursor.start().0, item, item_constraint, cx);
+                leading_overdraw += element.size().y();
+                rendered_items.push_front(ListItem::Rendered(element));
+            } else {
+                break;
+            }
+        }
+
+        let new_rendered_range = cursor.start().0..(cursor.start().0 + rendered_items.len());
+
+        let mut cursor = old_items.cursor::<Count>();
+
+        if state.rendered_range.start < new_rendered_range.start {
+            new_items.push_tree(
+                cursor.slice(&Count(state.rendered_range.start), Bias::Right, &()),
+                &(),
+            );
+            let remove_to = state.rendered_range.end.min(new_rendered_range.start);
+            while cursor.start().0 < remove_to {
+                new_items.push(cursor.item().unwrap().remove(), &());
+                cursor.next(&());
+            }
+        }
+        new_items.push_tree(
+            cursor.slice(&Count(new_rendered_range.start), Bias::Right, &()),
+            &(),
+        );
+
+        new_items.extend(rendered_items, &());
+        cursor.seek(&Count(new_rendered_range.end), Bias::Right, &());
+
+        if new_rendered_range.end < state.rendered_range.start {
+            new_items.push_tree(
+                cursor.slice(&Count(state.rendered_range.start), Bias::Right, &()),
+                &(),
+            );
+        }
+        while cursor.start().0 < state.rendered_range.end {
+            new_items.push(cursor.item().unwrap().remove(), &());
+            cursor.next(&());
+        }
+
+        new_items.push_tree(cursor.suffix(&()), &());
+
+        state.items = new_items;
+        state.rendered_range = new_rendered_range;
+        state.last_layout_width = Some(size.x());
+        (size, scroll_top)
+    }
+
+    fn paint(
+        &mut self,
+        bounds: RectF,
+        visible_bounds: RectF,
+        scroll_top: &mut ListOffset,
+        cx: &mut PaintContext,
+    ) {
+        cx.scene.push_layer(Some(bounds));
+
+        let state = &mut *self.state.0.borrow_mut();
+        for (mut element, origin) in state.visible_elements(bounds, scroll_top) {
+            element.paint(origin, visible_bounds, cx);
+        }
+
+        cx.scene.pop_layer();
+    }
+
+    fn dispatch_event(
+        &mut self,
+        event: &Event,
+        bounds: RectF,
+        scroll_top: &mut ListOffset,
+        _: &mut (),
+        cx: &mut EventContext,
+    ) -> bool {
+        let mut handled = false;
+
+        let mut state = self.state.0.borrow_mut();
+        let mut item_origin = bounds.origin() - vec2f(0., scroll_top.offset_in_item);
+        let mut cursor = state.items.cursor::<Count>();
+        let mut new_items = cursor.slice(&Count(scroll_top.item_ix), Bias::Right, &());
+        while let Some(item) = cursor.item() {
+            if item_origin.y() > bounds.max_y() {
+                break;
+            }
+
+            if let ListItem::Rendered(element) = item {
+                let prev_notify_count = cx.notify_count();
+                let mut element = element.clone();
+                handled = element.dispatch_event(event, cx) || handled;
+                item_origin.set_y(item_origin.y() + element.size().y());
+                if cx.notify_count() > prev_notify_count {
+                    new_items.push(ListItem::Unrendered, &());
+                    self.invalidated_elements.push(element);
+                } else {
+                    new_items.push(item.clone(), &());
+                }
+                cursor.next(&());
+            } else {
+                unreachable!();
+            }
+        }
+
+        new_items.push_tree(cursor.suffix(&()), &());
+        drop(cursor);
+        state.items = new_items;
+
+        match event {
+            Event::ScrollWheel {
+                position,
+                delta,
+                precise,
+            } => {
+                if bounds.contains_point(*position) {
+                    if state.scroll(scroll_top, bounds.height(), *delta, *precise, cx) {
+                        handled = true;
+                    }
+                }
+            }
+            _ => {}
+        }
+
+        handled
+    }
+
+    fn debug(
+        &self,
+        bounds: RectF,
+        scroll_top: &Self::LayoutState,
+        _: &(),
+        cx: &DebugContext,
+    ) -> serde_json::Value {
+        let state = self.state.0.borrow_mut();
+        let visible_elements = state
+            .visible_elements(bounds, scroll_top)
+            .map(|e| e.0.debug(cx))
+            .collect::<Vec<_>>();
+        let visible_range = scroll_top.item_ix..(scroll_top.item_ix + visible_elements.len());
+        json!({
+            "visible_range": visible_range,
+            "visible_elements": visible_elements,
+            "scroll_top": state.logical_scroll_top.map(|top| (top.item_ix, top.offset_in_item)),
+        })
+    }
+}
+
+impl ListState {
+    pub fn new<F>(
+        element_count: usize,
+        orientation: Orientation,
+        overdraw: f32,
+        render_item: F,
+    ) -> Self
+    where
+        F: 'static + FnMut(usize, &mut LayoutContext) -> ElementBox,
+    {
+        let mut items = SumTree::new();
+        items.extend((0..element_count).map(|_| ListItem::Unrendered), &());
+        Self(Rc::new(RefCell::new(StateInner {
+            last_layout_width: None,
+            render_item: Box::new(render_item),
+            rendered_range: 0..0,
+            items,
+            logical_scroll_top: None,
+            orientation,
+            overdraw,
+            scroll_handler: None,
+        })))
+    }
+
+    pub fn reset(&self, element_count: usize) {
+        let state = &mut *self.0.borrow_mut();
+        state.rendered_range = 0..0;
+        state.logical_scroll_top = None;
+        state.items = SumTree::new();
+        state
+            .items
+            .extend((0..element_count).map(|_| ListItem::Unrendered), &());
+    }
+
+    pub fn splice(&self, old_range: Range<usize>, count: usize) {
+        let state = &mut *self.0.borrow_mut();
+
+        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 = 0.;
+            } else if old_range.end <= *item_ix {
+                *item_ix = *item_ix - (old_range.end - old_range.start) + count;
+            }
+        }
+
+        let new_end = old_range.start + count;
+        if old_range.start < state.rendered_range.start {
+            state.rendered_range.start =
+                new_end + state.rendered_range.start.saturating_sub(old_range.end);
+        }
+        if old_range.start < state.rendered_range.end {
+            state.rendered_range.end =
+                new_end + state.rendered_range.end.saturating_sub(old_range.end);
+        }
+
+        let mut old_heights = state.items.cursor::<Count>();
+        let mut new_heights = old_heights.slice(&Count(old_range.start), Bias::Right, &());
+        old_heights.seek_forward(&Count(old_range.end), Bias::Right, &());
+
+        new_heights.extend((0..count).map(|_| ListItem::Unrendered), &());
+        new_heights.push_tree(old_heights.suffix(&()), &());
+        drop(old_heights);
+        state.items = new_heights;
+    }
+
+    pub fn set_scroll_handler(
+        &mut self,
+        handler: impl FnMut(Range<usize>, &mut EventContext) + 'static,
+    ) {
+        self.0.borrow_mut().scroll_handler = Some(Box::new(handler))
+    }
+}
+
+impl StateInner {
+    fn render_item(
+        &mut self,
+        ix: usize,
+        existing_item: &ListItem,
+        constraint: SizeConstraint,
+        cx: &mut LayoutContext,
+    ) -> ElementRc {
+        if let ListItem::Rendered(element) = existing_item {
+            element.clone()
+        } else {
+            let mut element = (self.render_item)(ix, cx);
+            element.layout(constraint, cx);
+            element.into()
+        }
+    }
+
+    fn visible_range(&self, height: f32, scroll_top: &ListOffset) -> Range<usize> {
+        let mut cursor = self.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 visible_elements<'a>(
+        &'a self,
+        bounds: RectF,
+        scroll_top: &ListOffset,
+    ) -> impl Iterator<Item = (ElementRc, Vector2F)> + 'a {
+        let mut item_origin = bounds.origin() - vec2f(0., scroll_top.offset_in_item);
+        let mut cursor = self.items.cursor::<Count>();
+        cursor.seek(&Count(scroll_top.item_ix), Bias::Right, &());
+        std::iter::from_fn(move || {
+            while let Some(item) = cursor.item() {
+                if item_origin.y() > bounds.max_y() {
+                    break;
+                }
+
+                if let ListItem::Rendered(element) = item {
+                    let result = (element.clone(), item_origin);
+                    item_origin.set_y(item_origin.y() + element.size().y());
+                    cursor.next(&());
+                    return Some(result);
+                }
+
+                cursor.next(&());
+            }
+
+            None
+        })
+    }
+
+    fn scroll(
+        &mut self,
+        scroll_top: &ListOffset,
+        height: f32,
+        mut delta: Vector2F,
+        precise: bool,
+        cx: &mut EventContext,
+    ) -> bool {
+        if !precise {
+            delta *= 20.;
+        }
+
+        let scroll_max = (self.items.summary().height - height).max(0.);
+        let new_scroll_top = (self.scroll_top(scroll_top) - delta.y())
+            .max(0.)
+            .min(scroll_max);
+
+        if self.orientation == Orientation::Bottom && new_scroll_top == scroll_max {
+            self.logical_scroll_top = None;
+        } else {
+            let mut cursor = self.items.cursor::<ListItemSummary>();
+            cursor.seek(&Height(new_scroll_top), Bias::Right, &());
+            let item_ix = cursor.start().count;
+            let offset_in_item = new_scroll_top - cursor.start().height;
+            self.logical_scroll_top = Some(ListOffset {
+                item_ix,
+                offset_in_item,
+            });
+        }
+
+        if self.scroll_handler.is_some() {
+            let visible_range = self.visible_range(height, scroll_top);
+            self.scroll_handler.as_mut().unwrap()(visible_range, cx);
+        }
+        cx.notify();
+
+        true
+    }
+
+    fn scroll_top(&self, logical_scroll_top: &ListOffset) -> f32 {
+        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
+    }
+}
+
+impl ListItem {
+    fn remove(&self) -> Self {
+        match self {
+            ListItem::Unrendered => ListItem::Unrendered,
+            ListItem::Rendered(element) => ListItem::Removed(element.size().y()),
+            ListItem::Removed(height) => ListItem::Removed(*height),
+        }
+    }
+}
+
+impl sum_tree::Item for ListItem {
+    type Summary = ListItemSummary;
+
+    fn summary(&self) -> Self::Summary {
+        match self {
+            ListItem::Unrendered => ListItemSummary {
+                count: 1,
+                rendered_count: 0,
+                unrendered_count: 1,
+                height: 0.,
+            },
+            ListItem::Rendered(element) => ListItemSummary {
+                count: 1,
+                rendered_count: 1,
+                unrendered_count: 0,
+                height: element.size().y(),
+            },
+            ListItem::Removed(height) => ListItemSummary {
+                count: 1,
+                rendered_count: 0,
+                unrendered_count: 1,
+                height: *height,
+            },
+        }
+    }
+}
+
+impl sum_tree::Summary for ListItemSummary {
+    type Context = ();
+
+    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;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ListItemSummary> for Count {
+    fn add_summary(&mut self, summary: &'a ListItemSummary, _: &()) {
+        self.0 += summary.count;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ListItemSummary> for RenderedCount {
+    fn add_summary(&mut self, summary: &'a ListItemSummary, _: &()) {
+        self.0 += summary.rendered_count;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ListItemSummary> for UnrenderedCount {
+    fn add_summary(&mut self, summary: &'a ListItemSummary, _: &()) {
+        self.0 += summary.unrendered_count;
+    }
+}
+
+impl<'a> sum_tree::Dimension<'a, ListItemSummary> for Height {
+    fn add_summary(&mut self, summary: &'a ListItemSummary, _: &()) {
+        self.0 += summary.height;
+    }
+}
+
+impl<'a> sum_tree::SeekTarget<'a, ListItemSummary, ListItemSummary> for Count {
+    fn cmp(&self, other: &ListItemSummary, _: &()) -> std::cmp::Ordering {
+        self.0.partial_cmp(&other.count).unwrap()
+    }
+}
+
+impl<'a> sum_tree::SeekTarget<'a, ListItemSummary, ListItemSummary> for Height {
+    fn cmp(&self, other: &ListItemSummary, _: &()) -> std::cmp::Ordering {
+        self.0.partial_cmp(&other.height).unwrap()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::geometry::vector::vec2f;
+    use rand::prelude::*;
+    use std::env;
+
+    #[crate::test(self)]
+    fn test_layout(cx: &mut crate::MutableAppContext) {
+        let mut presenter = cx.build_presenter(0, 0.);
+        let constraint = SizeConstraint::new(vec2f(0., 0.), vec2f(100., 40.));
+
+        let elements = Rc::new(RefCell::new(vec![(0, 20.), (1, 30.), (2, 100.)]));
+        let state = ListState::new(elements.borrow().len(), Orientation::Top, 1000.0, {
+            let elements = elements.clone();
+            move |ix, _| {
+                let (id, height) = elements.borrow()[ix];
+                TestElement::new(id, height).boxed()
+            }
+        });
+
+        let mut list = List::new(state.clone());
+        let (size, _) = list.layout(constraint, &mut presenter.build_layout_context(false, cx));
+        assert_eq!(size, vec2f(100., 40.));
+        assert_eq!(
+            state.0.borrow().items.summary(),
+            ListItemSummary {
+                count: 3,
+                rendered_count: 3,
+                unrendered_count: 0,
+                height: 150.
+            }
+        );
+
+        state.0.borrow_mut().scroll(
+            &ListOffset {
+                item_ix: 0,
+                offset_in_item: 0.,
+            },
+            40.,
+            vec2f(0., -54.),
+            true,
+            &mut presenter.build_event_context(cx),
+        );
+        let (_, logical_scroll_top) =
+            list.layout(constraint, &mut presenter.build_layout_context(false, cx));
+        assert_eq!(
+            logical_scroll_top,
+            ListOffset {
+                item_ix: 2,
+                offset_in_item: 4.
+            }
+        );
+        assert_eq!(state.0.borrow().scroll_top(&logical_scroll_top), 54.);
+
+        elements.borrow_mut().splice(1..2, vec![(3, 40.), (4, 50.)]);
+        elements.borrow_mut().push((5, 60.));
+        state.splice(1..2, 2);
+        state.splice(4..4, 1);
+        assert_eq!(
+            state.0.borrow().items.summary(),
+            ListItemSummary {
+                count: 5,
+                rendered_count: 2,
+                unrendered_count: 3,
+                height: 120.
+            }
+        );
+
+        let (size, logical_scroll_top) =
+            list.layout(constraint, &mut presenter.build_layout_context(false, cx));
+        assert_eq!(size, vec2f(100., 40.));
+        assert_eq!(
+            state.0.borrow().items.summary(),
+            ListItemSummary {
+                count: 5,
+                rendered_count: 5,
+                unrendered_count: 0,
+                height: 270.
+            }
+        );
+        assert_eq!(
+            logical_scroll_top,
+            ListOffset {
+                item_ix: 3,
+                offset_in_item: 4.
+            }
+        );
+        assert_eq!(state.0.borrow().scroll_top(&logical_scroll_top), 114.);
+    }
+
+    #[crate::test(self, iterations = 10, seed = 0)]
+    fn test_random(cx: &mut crate::MutableAppContext, mut rng: StdRng) {
+        let operations = env::var("OPERATIONS")
+            .map(|i| i.parse().expect("invalid `OPERATIONS` variable"))
+            .unwrap_or(10);
+
+        let mut presenter = cx.build_presenter(0, 0.);
+        let mut next_id = 0;
+        let elements = Rc::new(RefCell::new(
+            (0..rng.gen_range(0..=20))
+                .map(|_| {
+                    let id = next_id;
+                    next_id += 1;
+                    (id, rng.gen_range(0..=200) as f32 / 2.0)
+                })
+                .collect::<Vec<_>>(),
+        ));
+        let orientation = *[Orientation::Top, Orientation::Bottom]
+            .choose(&mut rng)
+            .unwrap();
+        let overdraw = rng.gen_range(1..=100) as f32;
+        let state = ListState::new(elements.borrow().len(), orientation, overdraw, {
+            let elements = elements.clone();
+            move |ix, _| {
+                let (id, height) = elements.borrow()[ix];
+                TestElement::new(id, height).boxed()
+            }
+        });
+
+        let mut width = rng.gen_range(0..=2000) as f32 / 2.;
+        let mut height = rng.gen_range(0..=2000) as f32 / 2.;
+        log::info!("orientation: {:?}", orientation);
+        log::info!("overdraw: {}", overdraw);
+        log::info!("elements: {:?}", elements.borrow());
+        log::info!("size: ({:?}, {:?})", width, height);
+        log::info!("==================");
+
+        let mut last_logical_scroll_top = None;
+        for _ in 0..operations {
+            match rng.gen_range(0..=100) {
+                0..=29 if last_logical_scroll_top.is_some() => {
+                    let delta = vec2f(0., rng.gen_range(-overdraw..=overdraw));
+                    log::info!(
+                        "Scrolling by {:?}, previous scroll top: {:?}",
+                        delta,
+                        last_logical_scroll_top.unwrap()
+                    );
+                    state.0.borrow_mut().scroll(
+                        last_logical_scroll_top.as_ref().unwrap(),
+                        height,
+                        delta,
+                        true,
+                        &mut presenter.build_event_context(cx),
+                    );
+                }
+                30..=34 => {
+                    width = rng.gen_range(0..=2000) as f32 / 2.;
+                    log::info!("changing width: {:?}", width);
+                }
+                35..=54 => {
+                    height = rng.gen_range(0..=1000) as f32 / 2.;
+                    log::info!("changing height: {:?}", height);
+                }
+                _ => {
+                    let mut elements = elements.borrow_mut();
+                    let end_ix = rng.gen_range(0..=elements.len());
+                    let start_ix = rng.gen_range(0..=end_ix);
+                    let new_elements = (0..rng.gen_range(0..10))
+                        .map(|_| {
+                            let id = next_id;
+                            next_id += 1;
+                            (id, rng.gen_range(0..=200) as f32 / 2.)
+                        })
+                        .collect::<Vec<_>>();
+                    log::info!("splice({:?}, {:?})", start_ix..end_ix, new_elements);
+                    state.splice(start_ix..end_ix, new_elements.len());
+                    elements.splice(start_ix..end_ix, new_elements);
+                    for (ix, item) in state.0.borrow().items.cursor::<()>().enumerate() {
+                        if let ListItem::Rendered(element) = item {
+                            let (expected_id, _) = elements[ix];
+                            element.with_metadata(|metadata: Option<&usize>| {
+                                assert_eq!(*metadata.unwrap(), expected_id);
+                            });
+                        }
+                    }
+                }
+            }
+
+            let mut list = List::new(state.clone());
+            let (size, logical_scroll_top) = list.layout(
+                SizeConstraint::new(vec2f(0., 0.), vec2f(width, height)),
+                &mut presenter.build_layout_context(false, cx),
+            );
+            assert_eq!(size, vec2f(width, height));
+            last_logical_scroll_top = Some(logical_scroll_top);
+
+            let state = state.0.borrow();
+            log::info!("items {:?}", state.items.items(&()));
+
+            let scroll_top = state.scroll_top(&logical_scroll_top);
+            let rendered_top = (scroll_top - overdraw).max(0.);
+            let rendered_bottom = scroll_top + height + overdraw;
+            let mut item_top = 0.;
+
+            log::info!(
+                "rendered top {:?}, rendered bottom {:?}, scroll top {:?}",
+                rendered_top,
+                rendered_bottom,
+                scroll_top,
+            );
+
+            let mut first_rendered_element_top = None;
+            let mut last_rendered_element_bottom = None;
+            assert_eq!(state.items.summary().count, elements.borrow().len());
+            for (ix, item) in state.items.cursor::<()>().enumerate() {
+                match item {
+                    ListItem::Unrendered => {
+                        let item_bottom = item_top;
+                        assert!(item_bottom <= rendered_top || item_top >= rendered_bottom);
+                        item_top = item_bottom;
+                    }
+                    ListItem::Removed(height) => {
+                        let (id, expected_height) = elements.borrow()[ix];
+                        assert_eq!(
+                            *height, expected_height,
+                            "element {} height didn't match",
+                            id
+                        );
+                        let item_bottom = item_top + height;
+                        assert!(item_bottom <= rendered_top || item_top >= rendered_bottom);
+                        item_top = item_bottom;
+                    }
+                    ListItem::Rendered(element) => {
+                        let (expected_id, expected_height) = elements.borrow()[ix];
+                        element.with_metadata(|metadata: Option<&usize>| {
+                            assert_eq!(*metadata.unwrap(), expected_id);
+                        });
+                        assert_eq!(element.size().y(), expected_height);
+                        let item_bottom = item_top + element.size().y();
+                        first_rendered_element_top.get_or_insert(item_top);
+                        last_rendered_element_bottom = Some(item_bottom);
+                        assert!(item_bottom > rendered_top || item_top < rendered_bottom);
+                        item_top = item_bottom;
+                    }
+                }
+            }
+
+            match orientation {
+                Orientation::Top => {
+                    if let Some(first_rendered_element_top) = first_rendered_element_top {
+                        assert!(first_rendered_element_top <= scroll_top);
+                    }
+                }
+                Orientation::Bottom => {
+                    if let Some(last_rendered_element_bottom) = last_rendered_element_bottom {
+                        assert!(last_rendered_element_bottom >= scroll_top + height);
+                    }
+                }
+            }
+        }
+    }
+
+    struct TestElement {
+        id: usize,
+        size: Vector2F,
+    }
+
+    impl TestElement {
+        fn new(id: usize, height: f32) -> Self {
+            Self {
+                id,
+                size: vec2f(100., height),
+            }
+        }
+    }
+
+    impl Element for TestElement {
+        type LayoutState = ();
+        type PaintState = ();
+
+        fn layout(&mut self, _: SizeConstraint, _: &mut LayoutContext) -> (Vector2F, ()) {
+            (self.size, ())
+        }
+
+        fn paint(&mut self, _: RectF, _: RectF, _: &mut (), _: &mut PaintContext) {
+            todo!()
+        }
+
+        fn dispatch_event(
+            &mut self,
+            _: &Event,
+            _: RectF,
+            _: &mut (),
+            _: &mut (),
+            _: &mut EventContext,
+        ) -> bool {
+            todo!()
+        }
+
+        fn debug(&self, _: RectF, _: &(), _: &(), _: &DebugContext) -> serde_json::Value {
+            self.id.into()
+        }
+
+        fn metadata(&self) -> Option<&dyn std::any::Any> {
+            Some(&self.id)
+        }
+    }
+}