Add compositor scrolling experiment

Co-authored-by: npub12gtutshhh76rx0jx697f32f9tffd4hhp3hx58fp4x6u4uemkm7sqf8f757 <5217c5c2f7bfb4333e46d17c98a9255a52dadee18dcd43a43536b95e6776dfa0@sprout-oss.stage.blox.sqprod.co>
Signed-off-by: npub12gtutshhh76rx0jx697f32f9tffd4hhp3hx58fp4x6u4uemkm7sqf8f757 <5217c5c2f7bfb4333e46d17c98a9255a52dadee18dcd43a43536b95e6776dfa0@sprout-oss.stage.blox.sqprod.co>

desktop/src/app/AppShell.tsx

@@ -96,7 +96,10 @@ const LazySettingsScreen = React.lazy(async () => {
 export function AppShell() {
   useWebviewZoomShortcuts();
   useTauriWindowDrag();
-  useWebviewScrollBoundaryLock();
+  const compositorTimelineScrollEnabled = useFeatureEnabled(
+    "compositorTimelineScroll",
+  );
+  useWebviewScrollBoundaryLock(!compositorTimelineScrollEnabled);
 
   const workspacesHook = useWorkspaces();
   const workspaceRailEnabled = useFeatureEnabled("workspaceRail");

desktop/src/shared/hooks/useWebviewScrollBoundaryLock.ts

@@ -80,8 +80,12 @@ function isConversationScroller(element: HTMLElement) {
  * remain; every other boundary — including all horizontal ones — is locked and
  * cannot chain to the viewport.
  */
-export function useWebviewScrollBoundaryLock() {
+export function useWebviewScrollBoundaryLock(enabled = true) {
   React.useEffect(() => {
+    if (!enabled) {
+      return;
+    }
+
     function handleWheel(event: WheelEvent) {
       if (event.defaultPrevented || event.ctrlKey) {
         return;
@@ -137,5 +141,5 @@ export function useWebviewScrollBoundaryLock() {
     return () => {
       window.removeEventListener("wheel", handleWheel, { capture: true });
     };
-  }, []);
+  }, [enabled]);
 }

desktop/tests/e2e/overscroll-boundary.spec.ts

@@ -28,7 +28,38 @@ async function dispatchWheelPrevented(
 }
 
 test.beforeEach(async ({ page }) => {
-  await installMockBridge(page);
+  // Boundary-lock contracts exercise the stable/default path. Most E2E suites
+  // opt every preview feature in, but compositorTimelineScroll intentionally
+  // removes this listener so trackpad scrolling can stay off the main thread.
+  await installMockBridge(page, undefined, { seedPreviewFeatures: false });
+});
+
+test("compositor timeline scrolling leaves timeline wheel events unhandled", async ({
+  page,
+}) => {
+  await page.addInitScript(() => {
+    localStorage.setItem(
+      "buzz-feature-overrides-v1",
+      JSON.stringify({ compositorTimelineScroll: true }),
+    );
+  });
+  await page.goto("/");
+  await page.getByTestId("channel-general").click();
+  await expect(page.getByTestId("message-timeline")).toBeVisible();
+
+  // The diagnostic path removes the global non-passive listener rather than
+  // merely returning early for conversations. The fixed-height shell and CSS
+  // overscroll backstop remain; JS no longer blocks timeline scrolling.
+  await expect(
+    dispatchWheelPrevented(page, '[data-testid="message-timeline"]', {
+      deltaY: -120,
+    }),
+  ).resolves.toBe(false);
+  await expect(
+    dispatchWheelPrevented(page, '[data-testid="message-timeline"]', {
+      deltaX: -120,
+    }),
+  ).resolves.toBe(false);
 });
 
 test("locks viewport rubber-band outside conversation scrollers", async ({

preview-features.json

@@ -40,6 +40,14 @@
       "platforms": [
         "desktop"
       ]
+    },
+    {
+      "id": "compositorTimelineScroll",
+      "name": "Compositor timeline scrolling",
+      "description": "Lets the webview handle timeline trackpad gestures without a global blocking wheel listener",
+      "platforms": [
+        "desktop"
+      ]
     }
   ]
 }