Maintain logical scroll top when splicing List elements

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

gpui/src/elements/list.rs

@@ -261,6 +261,15 @@ impl ListState {
     pub fn splice(&self, old_range: Range<usize>, count: usize) {
         let state = &mut *self.0.borrow_mut();
 
+        if let Some((ix, offset)) = state.scroll_top.as_mut() {
+            if old_range.contains(ix) {
+                *ix = old_range.start;
+                *offset = 0.;
+            } else if old_range.end <= *ix {
+                *ix = *ix - (old_range.end - old_range.start) + count;
+            }
+        }
+
         let mut old_heights = state.heights.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, &());
@@ -439,7 +448,7 @@ mod tests {
     fn test_layout(cx: &mut crate::MutableAppContext) {
         let mut presenter = cx.build_presenter(0, 0.);
 
-        let mut elements = vec![20., 30., 10.];
+        let mut elements = vec![20., 30., 100.];
         let state = ListState::new(elements.len(), Orientation::Top);
 
         let mut list = List::new(
@@ -458,10 +467,20 @@ mod tests {
             ElementHeightSummary {
                 count: 3,
                 pending_count: 0,
-                height: 60.
+                height: 150.
             }
         );
 
+        state.0.borrow_mut().scroll(
+            Default::default(),
+            vec2f(0., 54.),
+            true,
+            size.y(),
+            &mut presenter.build_event_context(cx),
+        );
+        assert_eq!(state.0.borrow().scroll_top, Some((2, 4.)));
+        assert_eq!(state.0.borrow().scroll_top(size.y()), 54.);
+
         elements.splice(1..2, vec![40., 50.]);
         elements.push(60.);
         state.splice(1..2, 2);
@@ -471,7 +490,7 @@ mod tests {
             ElementHeightSummary {
                 count: 5,
                 pending_count: 3,
-                height: 30.
+                height: 120.
             }
         );
 
@@ -491,9 +510,11 @@ mod tests {
             ElementHeightSummary {
                 count: 5,
                 pending_count: 0,
-                height: 180.
+                height: 270.
             }
         );
+        assert_eq!(state.0.borrow().scroll_top, Some((3, 4.)));
+        assert_eq!(state.0.borrow().scroll_top(size.y()), 114.);
     }
 
     fn item(height: f32) -> ElementBox {

gpui/src/presenter.rs

@@ -160,15 +160,7 @@ impl Presenter {
                 _ => {}
             }
 
-            let mut event_cx = EventContext {
-                rendered_views: &mut self.rendered_views,
-                dispatched_actions: Default::default(),
-                font_cache: &self.font_cache,
-                text_layout_cache: &self.text_layout_cache,
-                view_stack: Default::default(),
-                invalidated_views: Default::default(),
-                app: cx,
-            };
+            let mut event_cx = self.build_event_context(cx);
             event_cx.dispatch_event(root_view_id, &event);
 
             let invalidated_views = event_cx.invalidated_views;
@@ -183,6 +175,21 @@ impl Presenter {
         }
     }
 
+    pub fn build_event_context<'a>(
+        &'a mut self,
+        cx: &'a mut MutableAppContext,
+    ) -> EventContext<'a> {
+        EventContext {
+            rendered_views: &mut self.rendered_views,
+            dispatched_actions: Default::default(),
+            font_cache: &self.font_cache,
+            text_layout_cache: &self.text_layout_cache,
+            view_stack: Default::default(),
+            invalidated_views: Default::default(),
+            app: cx,
+        }
+    }
+
     pub fn debug_elements(&self, cx: &AppContext) -> Option<json::Value> {
         cx.root_view_id(self.window_id)
             .and_then(|root_view_id| self.rendered_views.get(&root_view_id))
@@ -274,8 +281,9 @@ impl<'a> EventContext<'a> {
     }
 
     pub fn notify(&mut self) {
-        self.invalidated_views
-            .insert(*self.view_stack.last().unwrap());
+        if let Some(view_id) = self.view_stack.last() {
+            self.invalidated_views.insert(*view_id);
+        }
     }
 }