Reset panel saved size when default_size changes (#61855)

Closes #59235

When a panel's `default_size` changes (e.g. `git_panel.default_width` is
edited in settings), the persisted per-workspace size is now cleared so
the new default takes effect immediately. Previously the saved dragged
size took precedence forever, making `default_width` effectively useless
after the first manual resize.

This fix is at the Dock level (`crates/workspace/src/dock.rs`), so it
applies to all panels — Git Panel, Project Panel, Terminal Panel, etc.

Release Notes:

- Fixed panel `default_width` setting being ignored after manually
resizing the panel


https://github.com/user-attachments/assets/97ef69b2-387f-4e29-bc4c-0f336f24eb0b

crates/workspace/src/dock.rs

@@ -364,7 +364,7 @@ pub struct PanelSizeState {
 struct PanelEntry {
     panel: Arc<dyn PanelHandle>,
     size_state: PanelSizeState,
-    _subscriptions: [Subscription; 3],
+    _subscriptions: [Subscription; 4],
 }
 
 pub struct PanelButtons {
@@ -671,6 +671,29 @@ impl Dock {
                         .ok();
                 }
             }),
+            {
+                let panel = panel.clone();
+                let mut last_default_size = panel.read(cx).default_size(window, cx);
+
+                cx.observe_global_in::<SettingsStore>(window, move |this, window, cx| {
+                    let default_size = panel.read(cx).default_size(window, cx);
+                    if default_size == last_default_size {
+                        return;
+                    }
+                    last_default_size = default_size;
+
+                    let panel_id = Entity::entity_id(&panel);
+                    if let Some(entry) = this
+                        .panel_entries
+                        .iter_mut()
+                        .find(|entry| entry.panel.panel_id() == panel_id)
+                    {
+                        entry.size_state.size = None;
+                        entry.panel.size_state_changed(window, cx);
+                        cx.notify();
+                    }
+                })
+            },
             cx.subscribe_in(
                 &panel,
                 window,

crates/workspace/src/workspace.rs

@@ -14557,6 +14557,75 @@ mod tests {
         }
     }
 
+    #[gpui::test]
+    async fn test_default_size_change_resets_saved_size(cx: &mut gpui::TestAppContext) {
+        init_test(cx);
+        let fs = FakeFs::new(cx.executor());
+
+        let project = Project::test(fs.clone(), [], cx).await;
+        let (multi_workspace, cx) =
+            cx.add_window_view(|window, cx| MultiWorkspace::test_new(project, window, cx));
+        let workspace = multi_workspace.read_with(cx, |mw, _| mw.workspace().clone());
+
+        workspace.update(cx, |workspace, _cx| {
+            workspace.set_random_database_id();
+            workspace.bounds.size.width = px(800.);
+        });
+
+        let panel = workspace.update_in(cx, |workspace, window, cx| {
+            let panel = cx.new(|cx| TestPanel::new(DockPosition::Left, 100, cx));
+            workspace.add_panel(panel.clone(), window, cx);
+            workspace.toggle_dock(DockPosition::Left, window, cx);
+            panel
+        });
+
+        workspace.update_in(cx, |workspace, window, cx| {
+            workspace.resize_left_dock(px(350.), window, cx);
+        });
+
+        cx.run_until_parked();
+
+        let dock_size = workspace.read_with(cx, |workspace, cx| {
+            workspace
+                .left_dock()
+                .read(cx)
+                .panel::<TestPanel>()
+                .and_then(|panel| {
+                    workspace
+                        .left_dock()
+                        .read(cx)
+                        .stored_panel_size_state(&panel)
+                })
+                .and_then(|s| s.size)
+        });
+        assert_eq!(dock_size, Some(px(350.)));
+
+        panel.update(cx, |panel, cx| {
+            panel.default_size = px(500.);
+            cx.update_global::<SettingsStore, _>(|_, _| {});
+        });
+
+        cx.run_until_parked();
+
+        let dock_size = workspace.read_with(cx, |workspace, cx| {
+            workspace
+                .left_dock()
+                .read(cx)
+                .panel::<TestPanel>()
+                .and_then(|panel| {
+                    workspace
+                        .left_dock()
+                        .read(cx)
+                        .stored_panel_size_state(&panel)
+                })
+                .and_then(|s| s.size)
+        });
+        assert_eq!(
+            dock_size, None,
+            "saved size should be cleared after default_size changes"
+        );
+    }
+
     #[gpui::test]
     async fn test_flexible_panel_left_dock_sizing(cx: &mut gpui::TestAppContext) {
         init_test(cx);