feat: add scrollbar for virtualized lists

docs/CHECKLIST.md

@@ -9,18 +9,16 @@ is the first downstream consumer; its screens drive the order.
 
 ## Next
 
-- [ ] **Command palette** (M12) — filtered actions; the matcher stays
-      caller-owned.
+- [ ] **Scrollbar** (M13) — a simple way to show a scrollbar on gpui's
+      `list` and `uniform_list`.
     - [x] API sketch
-    - [x] Implementation (theme-baked, no entry motion per AGENTS.md)
+    - [x] Implementation (theme-baked, overlays virtualized lists)
     - [x] Tests
     - [x] Gallery panel
     - [ ] Review
 
 ## Queue — to follow
 
-- [ ] **VirtualList** (M13, `data`) — thin `uniform_list` binding for commit
-      histories and the inbox.
 - [ ] **Code view** (M14, opens `crates/editor`) — tree-sitter highlighting.
 - [ ] **Diff view** (M15, `editor`).
 - [ ] **Markdown view** (M16, `editor`) — only if a consumer confirms.
@@ -62,6 +60,7 @@ is the first downstream consumer; its screens drive the order.
 - [x] Status dot — colored state bead, Badge's variant vocabulary, optional
       screen-reader label
 - [x] Combobox — type-to-filter picker, create / keep / revert blur modes
+- [x] Command palette — filtered actions; the matcher stays caller-owned
 
 ### data (`crates/data`)