fix(person-images): prevent flash of previous page's images (#1315)

Scope the grid to the active cluster so the shared `images` slice left by the
previous page can no longer paint before the person's photos load. Track which
cluster the slice holds via `loadedClusterId`; until it syncs, render from the
`['person-images', clusterId]` query data, which is always scoped to the active
id. Reading the slice once synced keeps the optimistic favourite toggle working.

Adds a regression test that fails when the previous page's images render.

Author: tanmaysachann

frontend/src/pages/PersonImages/PersonImages.tsx

@@ -22,6 +22,11 @@ export const PersonImages = () => {
   const images = useSelector(selectImages);
   const [clusterName, setClusterName] = useState<string>('random_name');
   const [isEditing, setIsEditing] = useState<boolean>(false);
+  // Which cluster the shared `images` slice currently holds. Keyed on clusterId
+  // (not `isLoading`) so it also covers person A -> person B when B is cached.
+  const [loadedClusterId, setLoadedClusterId] = useState<string | undefined>(
+    undefined,
+  );
   const { data, isLoading, isSuccess, isError } = usePictoQuery({
     queryKey: ['person-images', clusterId],
     queryFn: async () => fetchClusterImages({ clusterId: clusterId || '' }),
@@ -42,9 +47,18 @@ export const PersonImages = () => {
       const images = (res?.images || []) as Image[];
       dispatch(setImages(images));
       setClusterName(res?.cluster_name || 'random_name');
+      setLoadedClusterId(clusterId);
       dispatch(hideLoader());
     }
-  }, [data, isSuccess, isError, isLoading, dispatch]);
+  }, [data, isSuccess, isError, isLoading, dispatch, clusterId]);
+
+  // Until the slice is synced for this cluster it still holds the previous
+  // page's images (issue #1315). Fall back to the cluster-scoped query data for
+  // that window; read the slice once synced so favourite toggles still work.
+  const personImages =
+    loadedClusterId === clusterId
+      ? images
+      : (((data?.data as any)?.images as Image[]) ?? []);
 
   const handleEditName = () => {
     setClusterName(clusterName);
@@ -109,7 +123,7 @@ export const PersonImages = () => {
       </div>
       <h1 className="mb-6 text-2xl font-bold">{clusterName}</h1>
       <div className="grid grid-cols-1 gap-4 sm:grid-cols-2 md:grid-cols-3 lg:grid-cols-4 xl:grid-cols-5">
-        {images.map((image, index) => (
+        {personImages.map((image, index) => (
           <ImageCard
             key={image.id}
             image={image}
@@ -121,7 +135,7 @@ export const PersonImages = () => {
       </div>
 
       {/* Media Viewer Modal */}
-      {isImageViewOpen && <MediaView images={images} />}
+      {isImageViewOpen && <MediaView images={personImages} />}
     </div>
   );
 };

frontend/src/pages/__tests__/PersonImages.test.tsx

@@ -0,0 +1,102 @@
+import { render, screen, waitFor } from '@/test-utils';
+import { Routes, Route } from 'react-router';
+import { PersonImages } from '@/pages/PersonImages/PersonImages';
+import type { Image } from '@/types/Media';
+import * as apiFunctions from '@/api/api-functions';
+
+// ImageCard resolves thumbnails through Tauri's convertFileSrc; return the path
+// unchanged so we can assert which person's images are in the DOM by their src.
+jest.mock('@tauri-apps/api/core', () => ({
+  invoke: jest.fn().mockResolvedValue(null),
+  convertFileSrc: (path: string) => path,
+}));
+
+jest.mock('@/api/api-functions', () => ({
+  fetchClusterImages: jest.fn(),
+  renameCluster: jest.fn(),
+}));
+
+const fetchClusterImages = apiFunctions.fetchClusterImages as jest.Mock;
+
+const makeImage = (id: string, path: string): Image => ({
+  id,
+  path,
+  thumbnailPath: path,
+  folder_id: 'folder',
+  isTagged: true,
+  isFavourite: false,
+  tags: [],
+});
+
+// "Person A" stands in for whatever the previous page (e.g. the Home gallery)
+// left in the shared `images` slice; "Person B" is the cluster we navigate to.
+const personAImages = [
+  makeImage('a1', '/personA/1.jpg'),
+  makeImage('a2', '/personA/2.jpg'),
+];
+const personBImages = [
+  makeImage('b1', '/personB/1.jpg'),
+  makeImage('b2', '/personB/2.jpg'),
+];
+
+const renderPerson = (clusterId: string) =>
+  render(
+    <Routes>
+      <Route path="/person/:clusterId" element={<PersonImages />} />
+    </Routes>,
+    {
+      // Pre-seed the shared slice with the previous page's images. This is the
+      // exact condition that produced the flash described in issue #1315.
+      preloadedState: {
+        images: { images: personAImages, currentViewIndex: -1 },
+      },
+      initialRoutes: [`/person/${clusterId}`],
+    },
+  );
+
+const imageSrcs = (container: HTMLElement) =>
+  Array.from(container.querySelectorAll('img')).map((img) =>
+    img.getAttribute('src'),
+  );
+
+const hasPersonA = (container: HTMLElement) =>
+  imageSrcs(container).some((src) => src?.startsWith('/personA/'));
+const hasPersonB = (container: HTMLElement) =>
+  imageSrcs(container).some((src) => src?.startsWith('/personB/'));
+
+beforeEach(() => {
+  fetchClusterImages.mockReset();
+  fetchClusterImages.mockImplementation(async ({ clusterId }) => ({
+    success: true,
+    data: {
+      images: clusterId === 'B' ? personBImages : personAImages,
+      cluster_name: clusterId === 'B' ? 'Bob' : 'Alice',
+    },
+  }));
+});
+
+describe('PersonImages (issue #1315: no flash of the previous page)', () => {
+  test('never paints the stale slice images, on the first frame or after', async () => {
+    const { container } = renderPerson('B');
+
+    // First frame: before the cluster query resolves, the shared slice still
+    // holds Person A's images. The grid must not paint them, otherwise the user
+    // sees the flash. (This assertion fails against the pre-fix code.)
+    expect(hasPersonA(container)).toBe(false);
+
+    // Let the query resolve, then confirm Person A never appears at any point.
+    await waitFor(() => {
+      expect(hasPersonB(container)).toBe(true);
+    });
+    expect(hasPersonA(container)).toBe(false);
+  });
+
+  test("renders the navigated cluster's images and name once loaded", async () => {
+    const { container } = renderPerson('B');
+
+    expect(await screen.findByText('Bob')).toBeInTheDocument();
+    await waitFor(() => {
+      expect(hasPersonB(container)).toBe(true);
+    });
+  });
+});