fix(diff): give the diff overlay the scrollbar every other scroll area has

A working tree can run to thousands of lines here, and the overlay was
the one long document in the app with nothing to say how far it went or
where in it you were. Nine scroll areas already wrap in the shared
helper; this one had not even been given a ScrollHandle.

src/ui/diff_overlay.rs

@@ -31,6 +31,7 @@ pub(crate) struct DiffOverlayState {
     pub(crate) loading: bool,
     pub(crate) expanded: HashMap<String, bool>,
     pub(crate) focus: Option<String>,
+    pub(crate) scroll: gpui::ScrollHandle,
 }
 
 impl Tty7App {
@@ -97,6 +98,7 @@ impl Tty7App {
             loading: false,
             expanded: HashMap::new(),
             focus,
+            scroll: gpui::ScrollHandle::new(),
         });
         window.focus(&focus_handle, cx);
         self.spawn_diff_probe(cx);
@@ -263,9 +265,13 @@ impl Tty7App {
             DiffLoad::Ready(snap) if empty_snapshot(snap) => {
                 self.diff_message(t(L10nKey::DiffWorkingTreeClean), cx)
             }
-            DiffLoad::Ready(snap) => {
-                self.diff_file_list(snap, &overlay.expanded, focused_file(snap, overlay), cx)
-            }
+            DiffLoad::Ready(snap) => self.diff_file_list(
+                snap,
+                &overlay.expanded,
+                focused_file(snap, overlay),
+                &overlay.scroll,
+                cx,
+            ),
         };
 
         let header = self.diff_header(overlay, window, cx);
@@ -455,6 +461,7 @@ impl Tty7App {
         snap: &DiffSnapshot,
         expanded: &HashMap<String, bool>,
         focused: Option<usize>,
+        scroll: &gpui::ScrollHandle,
         cx: &mut Context<Self>,
     ) -> AnyElement {
         let stats = snap.stats();
@@ -493,13 +500,19 @@ impl Tty7App {
         if focused.is_none() && !snap.untracked.is_empty() {
             list = list.child(self.diff_untracked_section(snap, cx));
         }
-        div()
-            .id("diff-overlay-scroll")
-            .flex_1()
-            .min_h_0()
-            .overflow_y_scroll()
-            .child(list)
-            .into_any_element()
+        // A whole working tree can scroll past here with nothing to say how
+        // far it runs or where in it you are — the one long document in the
+        // app without the bar every other scroll area has.
+        crate::ui::scrollbar::with_vertical_scrollbar(
+            "diff-overlay-scrollbar",
+            div()
+                .id("diff-overlay-scroll")
+                .size_full()
+                .overflow_y_scroll()
+                .track_scroll(scroll)
+                .child(list),
+            scroll,
+        )
     }
 
     fn diff_oversized_notice(