gpui: Add item_is_above_viewport and item_is_below_viewport APIs to ListState (#58061)

In prep for handling the above-viewport case in
https://github.com/zed-industries/zed/pull/57632, which currently only
handles below case.

This PR adds `ListState::item_is_above_viewport` and
`ListState::item_is_below_viewport` methods, which report whether a
given list item is entirely outside the current viewport. Both return
`None` when the list has not measured enough layout to answer.

Release Notes:

- N/A

crates/gpui/src/elements/list.rs

@@ -741,6 +741,44 @@ impl ListState {
     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 {
+            // Rows before the logical scroll top have no item bounds, but
+            // their position relative to the viewport is known from scroll state.
+            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 {
+            // Rows before the logical scroll top have no item bounds, but
+            // their position relative to the viewport is known from scroll state.
+            return Some(false);
+        }
+
+        let item_bounds = self.bounds_for_item(ix)?;
+        Some(item_bounds.top() >= viewport_bounds.bottom())
+    }
 }
 
 impl StateInner {
@@ -1644,6 +1682,114 @@ mod test {
         assert_eq!(offset.offset_in_item, px(0.));
     }
 
+    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_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_measure_all_after_width_change(cx: &mut TestAppContext) {
         let cx = cx.add_empty_window();