project panel: Respect y offset of a click-and-drag on a scrollbar (#13506)

Previously we've always snapped the scrollbar to the cursor position,
without regard for the position of the thumb.



Release Notes:

- N/A

crates/project_panel/src/project_panel.rs

@@ -72,7 +72,7 @@ pub struct ProjectPanel {
     width: Option<Pixels>,
     pending_serialization: Task<Option<()>>,
     show_scrollbar: bool,
-    is_dragging_scrollbar: Rc<Cell<bool>>,
+    scrollbar_drag_thumb_offset: Rc<Cell<Option<f32>>>,
     hide_scrollbar_task: Option<Task<()>>,
 }
 
@@ -289,7 +289,7 @@ impl ProjectPanel {
                 pending_serialization: Task::ready(None),
                 show_scrollbar: !Self::should_autohide_scrollbar(cx),
                 hide_scrollbar_task: None,
-                is_dragging_scrollbar: Default::default(),
+                scrollbar_drag_thumb_offset: Default::default(),
             };
             this.update_visible_entries(None, cx);
 
@@ -2231,7 +2231,7 @@ impl ProjectPanel {
 
         let height = scroll_handle
             .last_item_height
-            .filter(|_| self.show_scrollbar || self.is_dragging_scrollbar.get())?;
+            .filter(|_| self.show_scrollbar || self.scrollbar_drag_thumb_offset.get().is_some())?;
 
         let total_list_length = height.0 as f64 * items_count as f64;
         let current_offset = scroll_handle.base_handle.offset().y.0.min(0.).abs() as f64;
@@ -2270,7 +2270,7 @@ impl ProjectPanel {
                 .on_mouse_up(
                     MouseButton::Left,
                     cx.listener(|this, _, cx| {
-                        if !this.is_dragging_scrollbar.get()
+                        if this.scrollbar_drag_thumb_offset.get().is_none()
                             && !this.focus_handle.contains_focused(cx)
                         {
                             this.hide_scrollbar(cx);
@@ -2293,7 +2293,7 @@ impl ProjectPanel {
                 .child(ProjectPanelScrollbar::new(
                     percentage as f32..end_offset as f32,
                     self.scroll_handle.clone(),
-                    self.is_dragging_scrollbar.clone(),
+                    self.scrollbar_drag_thumb_offset.clone(),
                     cx.view().clone().into(),
                     items_count,
                 )),