fix(feed): an optimistic write outlives only the server answers older than it

A reset wrote a position mask over the cached list, so a replay's saved
position stayed hidden: a reopen resumed at 0 and its first checkpoint wrote
over the progress. The reset now writes 0 into the cache, where the player's
closing patch writes the next one. Overrides carry their write order, and a
landing refetch drops only those written before it set out. The player
session ends after the modal unmounts, so a clip auto-marked watched as it
closes still refetches the list.

frontend/src/components/Feed.reset.dom.test.tsx

@@ -5,7 +5,7 @@
 // the next refetch (found in a UAT). The grid is replaced by a list that shows what Feed
 // hands it: each video's position, and a button that fires the reset.
 import { describe, it, expect, vi, beforeAll, afterEach } from "vitest";
-import { render, screen, fireEvent, waitFor, cleanup } from "@testing-library/react";
+import { render, screen, fireEvent, waitFor, cleanup, act } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
 vi.mock("react-i18next", async (importOriginal) => ({
@@ -35,6 +35,7 @@ vi.mock("./VirtualFeed", () => ({
 import Feed from "./Feed";
 import { api, DEFAULT_TAG_MODES, type FeedFilters, type Video } from "../lib/api";
 import { qk } from "../lib/queryKeys";
+import { patchCachedProgress } from "../lib/progressCache";
 
 beforeAll(() => {
   class Stub {
@@ -78,30 +79,52 @@ const halfWatched = {
   watch_url: "https://www.youtube.com/watch?v=v1",
 } as unknown as Video;
 
+function mount(clear: () => Promise<unknown>) {
+  vi.spyOn(api, "feed").mockResolvedValue({ items: [halfWatched], next_cursor: null } as never);
+  vi.spyOn(api, "feedSummary").mockResolvedValue({ count: 1, facets: {} } as never);
+  vi.spyOn(api, "clearState").mockImplementation(clear as never);
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  qc.setQueryData(qk.me(), { id: 1, can_write: true, prefs: {} });
+  render(
+    <QueryClientProvider client={qc}>
+      <Feed
+        filters={FILTERS}
+        setFilters={() => {}}
+        view="cards"
+        setView={() => {}}
+        canRead
+        ytSearch=""
+        onYtSearch={() => {}}
+        onExitYtSearch={() => {}}
+      />
+    </QueryClientProvider>,
+  );
+  return qc;
+}
+
 describe("Feed — resetting a half-watched video", () => {
   it("clears its resume position at once, not only its status", async () => {
-    vi.spyOn(api, "feed").mockResolvedValue({ items: [halfWatched], next_cursor: null } as never);
-    vi.spyOn(api, "feedSummary").mockResolvedValue({ count: 1, facets: {} } as never);
     // Never settles: what the card shows before the server answers is the optimistic layer alone.
-    vi.spyOn(api, "clearState").mockReturnValue(new Promise(() => {}) as never);
-    const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
-    qc.setQueryData(qk.me(), { id: 1, can_write: true, prefs: {} });
-    render(
-      <QueryClientProvider client={qc}>
-        <Feed
-          filters={FILTERS}
-          setFilters={() => {}}
-          view="cards"
-          setView={() => {}}
-          canRead
-          ytSearch=""
-          onYtSearch={() => {}}
-          onExitYtSearch={() => {}}
-        />
-      </QueryClientProvider>,
-    );
+    mount(() => new Promise(() => {}));
     await waitFor(() => expect(screen.getByText("v1@95")).toBeTruthy());
     fireEvent.click(screen.getByText("reset v1"));
     await waitFor(() => expect(screen.getByText("v1@0")).toBeTruthy());
   });
+
+  it("shows the position a replay after it saved, so a reopen resumes there", async () => {
+    // Reset, played to 40 s and closed: the player writes what the server stored into the cache.
+    // A reset mask over the cache kept showing 0, a reopen resumed at 0, and its first checkpoint
+    // wrote over the 40.
+    const qc = mount(() => Promise.resolve({ ok: true }));
+    fireEvent.click(await screen.findByText("reset v1"));
+    await screen.findByText("v1@0");
+    act(() => patchCachedProgress(qc, new Map([["v1", 40]])));
+    await waitFor(() => expect(screen.getByText("v1@40")).toBeTruthy());
+  });
+
+  it("puts the position back when the server refuses the reset", async () => {
+    mount(() => Promise.reject(new Error("down")));
+    fireEvent.click(await screen.findByText("reset v1"));
+    await waitFor(() => expect(screen.getByText("v1@95")).toBeTruthy());
+  });
 });