Fix project panel follow focus issues (#60974)

Closes https://github.com/zed-industries/zed/issues/60939
Closes https://github.com/zed-industries/zed/issues/60940

Release Notes:

- Fixed focus follows mouse in project panel's blank space

crates/gpui/src/window.rs

@@ -2723,6 +2723,7 @@ impl Window {
         self.next_frame.clear();
         let current_focus_path = self.rendered_frame.focus_path();
         let current_window_active = self.rendered_frame.window_active;
+        let mut focus_before_listeners = self.focus;
 
         if previous_focus_path != current_focus_path
             || previous_window_active != current_window_active
@@ -2731,6 +2732,11 @@ impl Window {
                 self.focus_lost_listeners
                     .clone()
                     .retain(&(), |listener| listener(self, cx));
+                // The focus-lost fallback (e.g. a workspace refocusing itself) may target
+                // an element that isn't part of the element tree, in which case scheduling
+                // a redraw below would dispatch focus-lost again, looping forever. Only
+                // track focus movement caused by the focus listeners.
+                focus_before_listeners = self.focus;
             }
 
             let event = WindowFocusEvent {
@@ -2755,6 +2761,13 @@ impl Window {
         self.reset_cursor_style(cx);
         self.refreshing = false;
         self.invalidator.set_phase(DrawPhase::None);
+        // Focus listeners may move focus (e.g. a dock forwarding focus to its active
+        // panel). `Window::focus` suppresses `refresh` while a draw is in progress, so
+        // schedule another frame here to render the new focus state and dispatch the
+        // resulting focus events.
+        if self.focus != focus_before_listeners {
+            self.refresh();
+        }
         self.needs_present.set(true);
 
         if let Some(draw_start) = draw_started_at {
@@ -6380,8 +6393,9 @@ pub fn outline(
 #[cfg(test)]
 mod tests {
     use crate::{
-        AppContext as _, Bounds, Context, IntoElement, ParentElement as _, Pixels, Render,
-        Styled as _, TestAppContext, Window, canvas, div, px, size,
+        AppContext as _, Bounds, Context, FocusHandle, InteractiveElement as _, IntoElement,
+        ParentElement as _, Pixels, Render, Styled as _, TestAppContext, Window, canvas, div, px,
+        size,
     };
     use std::{cell::Cell, rc::Rc};
 
@@ -6449,4 +6463,63 @@ mod tests {
 
         assert_eq!(child_bounds.get().size, size(px(300.), px(200.)));
     }
+
+    struct FocusForwarder {
+        a: FocusHandle,
+        b: FocusHandle,
+    }
+
+    impl Render for FocusForwarder {
+        fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
+            div()
+                .size_full()
+                .child(div().w(px(50.)).h(px(50.)).track_focus(&self.a))
+                .child(div().w(px(50.)).h(px(50.)).track_focus(&self.b))
+        }
+    }
+
+    /// When a focus listener moves focus again (e.g. a dock forwarding focus to its
+    /// active panel), the resulting focus events must be dispatched without waiting
+    /// for an unrelated redraw of the window.
+    #[gpui::test]
+    fn test_focus_moved_by_focus_listener_is_dispatched(cx: &mut TestAppContext) {
+        let b_focus_count = Rc::new(Cell::new(0));
+        let window = cx.add_window({
+            let b_focus_count = b_focus_count.clone();
+            move |window, cx| {
+                let a = cx.focus_handle();
+                let b = cx.focus_handle();
+                cx.on_focus(&a, window, |this: &mut FocusForwarder, window, cx| {
+                    let b = this.b.clone();
+                    window.focus(&b, cx);
+                })
+                .detach();
+                cx.on_focus(&b, window, move |_, _, _| {
+                    b_focus_count.set(b_focus_count.get() + 1);
+                })
+                .detach();
+                FocusForwarder { a, b }
+            }
+        });
+
+        window
+            .update(cx, |_, window, _| window.activate_window())
+            .unwrap();
+        cx.executor().run_until_parked();
+
+        window
+            .update(cx, |this, window, cx| {
+                let a = this.a.clone();
+                window.focus(&a, cx);
+            })
+            .unwrap();
+        cx.executor().run_until_parked();
+
+        window
+            .update(cx, |this, window, _| {
+                assert!(this.b.is_focused(window));
+            })
+            .unwrap();
+        assert_eq!(b_focus_count.get(), 1);
+    }
 }

crates/project_panel/src/project_panel.rs

@@ -74,8 +74,9 @@ use util::{
 };
 use workspace::{
     DraggedSelection, OpenInTerminal, OpenMode, OpenOptions, OpenVisible, PreviewTabsSettings,
-    SelectedEntry, SplitDirection, Workspace,
+    SelectedEntry, SplitDirection, Workspace, WorkspaceSettings,
     dock::{DockPosition, Panel, PanelEvent},
+    focus_follows_mouse::FocusFollowsMouse as _,
     notifications::{DetachAndPromptErr, NotifyResultExt, NotifyTaskExt},
 };
 use worktree::CreatedEntry;
@@ -7244,6 +7245,13 @@ impl Render for ProjectPanel {
                             div()
                                 .id("project-panel-blank-area")
                                 .block_mouse_except_scroll()
+                                // `block_mouse_except_scroll` prevents the dock's own
+                                // focus-follows-mouse hover handler from seeing this area,
+                                // so handle it here directly.
+                                .focus_follows_mouse(
+                                    WorkspaceSettings::get_global(cx).focus_follows_mouse,
+                                    cx,
+                                )
                                 .flex_grow_1()
                                 .on_scroll_wheel({
                                     let scroll_handle = self.scroll_handle.clone();

crates/project_panel/src/project_panel_tests.rs

@@ -10979,6 +10979,71 @@ async fn run_create_file_in_folded_path_case(
     }
 }
 
+#[gpui::test]
+async fn test_focus_follows_mouse_into_blank_area(cx: &mut gpui::TestAppContext) {
+    init_test_with_editor(cx);
+    cx.update(|cx| {
+        cx.update_global::<SettingsStore, _>(|store, cx| {
+            store.update_user_settings(cx, |settings| {
+                settings.workspace.focus_follows_mouse = Some(settings::FocusFollowsMouse {
+                    enabled: Some(true),
+                    debounce_ms: Some(100),
+                });
+            });
+        });
+    });
+
+    let fs = FakeFs::new(cx.executor());
+    fs.insert_tree(path!("/root"), json!({ "a.txt": "" })).await;
+
+    let project = Project::test(fs.clone(), [path!("/root").as_ref()], cx).await;
+    let window = cx.add_window(|window, cx| MultiWorkspace::test_new(project.clone(), window, cx));
+    let workspace = window
+        .read_with(cx, |mw, _| mw.workspace().clone())
+        .unwrap();
+    let cx = &mut VisualTestContext::from_window(window.into(), cx);
+    let panel = workspace.update_in(cx, |workspace, window, cx| {
+        let panel = ProjectPanel::new(workspace, window, cx);
+        workspace.add_panel(panel.clone(), window, cx);
+        workspace.open_panel::<ProjectPanel>(window, cx);
+        panel
+    });
+    cx.run_until_parked();
+
+    workspace
+        .update_in(cx, |workspace, window, cx| {
+            let worktree_id = workspace.worktrees(cx).next().unwrap().read(cx).id();
+            let project_path = ProjectPath {
+                worktree_id,
+                path: rel_path("a.txt").into(),
+            };
+            workspace.open_path(project_path, None, true, window, cx)
+        })
+        .await
+        .unwrap();
+    cx.run_until_parked();
+
+    panel.update_in(cx, |panel, window, cx| {
+        assert!(
+            !panel.focus_handle(cx).is_focused(window),
+            "Editor should be focused after opening a file"
+        );
+    });
+
+    // Hover over the blank space below the last entry in the project panel,
+    // which lives in the right dock by default.
+    cx.simulate_mouse_move(point(px(1800.), px(600.)), None, Modifiers::none());
+    cx.executor().advance_clock(Duration::from_millis(200));
+    cx.run_until_parked();
+
+    panel.update_in(cx, |panel, window, cx| {
+        assert!(
+            panel.focus_handle(cx).is_focused(window),
+            "Project panel should be focused after hovering the blank area below the entries"
+        );
+    });
+}
+
 pub(crate) fn init_test(cx: &mut TestAppContext) {
     cx.update(|cx| {
         let settings_store = SettingsStore::test(cx);