fix(feed): an override yields only to a refetch that set out after its write was answered

Send order let a refetch read before an unanswered mark committed and drop it. A reset's position 0 is written again after a refetch that set out before the server took the reset. The player-session refetch also waits for the player's closing checkpoint, which the player now tracks (progressFlushes); refetched first, the list put the older position back.

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

@@ -40,10 +40,14 @@ vi.mock("./VirtualFeed", () => ({
   ),
 }));
 // The player's progress hook marks a clip closed near its end watched from its unmount cleanup:
-// `watchOnUnmount` stands in for that.
-const player = vi.hoisted(() => ({ watchOnUnmount: false }));
+// `watchOnUnmount` stands in for that, and `flush` for its closing checkpoint, which it tracks.
+const player = vi.hoisted(() => ({
+  watchOnUnmount: false,
+  flush: null as Promise<unknown> | null,
+}));
 vi.mock("./PlayerModal", async () => {
   const { useEffect } = await import("react");
+  const { trackProgressFlush } = await import("../lib/progressCache");
   return {
     default: function Player({
       video,
@@ -57,6 +61,7 @@ vi.mock("./PlayerModal", async () => {
       useEffect(
         () => () => {
           if (player.watchOnUnmount) onState(video.id, "watched");
+          if (player.flush) trackProgressFlush(player.flush);
         },
         [video.id, onState],
       );
@@ -91,6 +96,7 @@ afterEach(() => {
   cleanup();
   vi.restoreAllMocks();
   player.watchOnUnmount = false;
+  player.flush = null;
 });
 
 const FILTERS: FeedFilters = {
@@ -203,6 +209,22 @@ describe("Feed — a video marked watched in the player", () => {
     await waitFor(() => expect(feed.mock.calls.length).toBeGreaterThan(before));
   });
 
+  it("waits for the player's closing checkpoint before it refetches", async () => {
+    // Refetched before that write landed, the list read the older position and, landing after the
+    // player's patch, put it back: a reopen resumed early.
+    const { feed } = mount();
+    let land: (v: unknown) => void = () => {};
+    player.flush = new Promise((r) => (land = r));
+    fireEvent.click(await screen.findByText("open v1"));
+    fireEvent.click(await screen.findByText("mark"));
+    const before = feed.mock.calls.length;
+    fireEvent.click(screen.getByText("close"));
+    await act(() => new Promise((r) => setTimeout(r, 30)));
+    expect(feed.mock.calls.length).toBe(before);
+    await act(async () => land(undefined));
+    await waitFor(() => expect(feed.mock.calls.length).toBeGreaterThan(before));
+  });
+
   it("does not refetch on a close with nothing changed", async () => {
     const { feed } = mount();
     fireEvent.click(await screen.findByText("open v1"));