project panel: scroll when drag-hovering over the edge of a list (#20207)

Closes [19554](https://github.com/zed-industries/zed/issues/19554)

Release Notes:

- Added auto-scrolling to project panel when a vertical edge of a panel
is hovered with a dragged entry.

crates/project_panel/src/project_panel.rs

@@ -63,6 +63,9 @@ pub struct ProjectPanel {
     fs: Arc<dyn Fs>,
     focus_handle: FocusHandle,
     scroll_handle: UniformListScrollHandle,
+    // An update loop that keeps incrementing/decrementing scroll offset while there is a dragged entry that's
+    // hovered over the start/end of a list.
+    hover_scroll_task: Option<Task<()>>,
     visible_entries: Vec<(WorktreeId, Vec<Entry>, OnceCell<HashSet<Arc<Path>>>)>,
     /// Maps from leaf project entry ID to the currently selected ancestor.
     /// Relevant only for auto-fold dirs, where a single project panel entry may actually consist of several
@@ -307,6 +310,7 @@ impl ProjectPanel {
             let scroll_handle = UniformListScrollHandle::new();
             let mut this = Self {
                 project: project.clone(),
+                hover_scroll_task: None,
                 fs: workspace.app_state().fs.clone(),
                 focus_handle,
                 visible_entries: Default::default(),
@@ -2544,6 +2548,7 @@ impl ProjectPanel {
                 ))
                 .on_drop(cx.listener(
                     move |this, external_paths: &ExternalPaths, cx| {
+                        this.hover_scroll_task.take();
                         this.last_external_paths_drag_over_entry = None;
                         this.marked_entries.clear();
                         this.drop_external_files(external_paths.paths(), entry_id, cx);
@@ -2563,6 +2568,7 @@ impl ProjectPanel {
                 style.bg(cx.theme().colors().drop_target_background)
             })
             .on_drop(cx.listener(move |this, selections: &DraggedSelection, cx| {
+                this.hover_scroll_task.take();
                 this.drag_onto(selections, entry_id, kind.is_file(), cx);
             }))
             .child(
@@ -3057,9 +3063,67 @@ impl Render for ProjectPanel {
                 .map(|(_, worktree_entries, _)| worktree_entries.len())
                 .sum();
 
+            fn handle_drag_move_scroll<T: 'static>(
+                this: &mut ProjectPanel,
+                e: &DragMoveEvent<T>,
+                cx: &mut ViewContext<ProjectPanel>,
+            ) {
+                if !e.bounds.contains(&e.event.position) {
+                    return;
+                }
+                this.hover_scroll_task.take();
+                let panel_height = e.bounds.size.height;
+                if panel_height <= px(0.) {
+                    return;
+                }
+
+                let event_offset = e.event.position.y - e.bounds.origin.y;
+                // How far along in the project panel is our cursor? (0. is the top of a list, 1. is the bottom)
+                let hovered_region_offset = event_offset / panel_height;
+
+                // We want the scrolling to be a bit faster when the cursor is closer to the edge of a list.
+                // These pixels offsets were picked arbitrarily.
+                let vertical_scroll_offset = if hovered_region_offset <= 0.05 {
+                    8.
+                } else if hovered_region_offset <= 0.15 {
+                    5.
+                } else if hovered_region_offset >= 0.95 {
+                    -8.
+                } else if hovered_region_offset >= 0.85 {
+                    -5.
+                } else {
+                    return;
+                };
+                let adjustment = point(px(0.), px(vertical_scroll_offset));
+                this.hover_scroll_task = Some(cx.spawn(move |this, mut cx| async move {
+                    loop {
+                        let should_stop_scrolling = this
+                            .update(&mut cx, |this, cx| {
+                                this.hover_scroll_task.as_ref()?;
+                                let handle = this.scroll_handle.0.borrow_mut();
+                                let offset = handle.base_handle.offset();
+
+                                handle.base_handle.set_offset(offset + adjustment);
+                                cx.notify();
+                                Some(())
+                            })
+                            .ok()
+                            .flatten()
+                            .is_some();
+                        if should_stop_scrolling {
+                            return;
+                        }
+                        cx.background_executor()
+                            .timer(Duration::from_millis(16))
+                            .await;
+                    }
+                }));
+            }
             h_flex()
                 .id("project-panel")
                 .group("project-panel")
+                .on_drag_move(cx.listener(handle_drag_move_scroll::<ExternalPaths>))
+                .on_drag_move(cx.listener(handle_drag_move_scroll::<DraggedSelection>))
                 .size_full()
                 .relative()
                 .on_hover(cx.listener(|this, hovered, cx| {
@@ -3291,6 +3355,7 @@ impl Render for ProjectPanel {
                         move |this, external_paths: &ExternalPaths, cx| {
                             this.last_external_paths_drag_over_entry = None;
                             this.marked_entries.clear();
+                            this.hover_scroll_task.take();
                             if let Some(task) = this
                                 .workspace
                                 .update(cx, |workspace, cx| {