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/lib/playerModalHooks.progress.dom.test.tsx

@@ -14,7 +14,7 @@ import { renderHook } from "@testing-library/react";
 import { QueryClient } from "@tanstack/react-query";
 import { createRef, type RefObject } from "react";
 import { useYtProgress } from "./playerModalHooks";
-import { patchCachedProgress } from "./progressCache";
+import { patchCachedProgress, progressFlushes } from "./progressCache";
 import { qk } from "./queryKeys";
 import { api, type Video } from "./api";
 import type { YTPlayer } from "./youtube";
@@ -113,6 +113,27 @@ describe("useYtProgress — the checkpoint reaches the cache", () => {
     expect(cachedPos(qc)).toBe(830);
   });
 
+  it("hands its closing checkpoint out until it has landed and been patched in", async () => {
+    // A list refetched on the player's close waits for it: read before it landed, the list held the
+    // older position and, landing after the patch, put it back.
+    const qc = new QueryClient();
+    seedFeed(qc, video());
+    atPosition(830, 3118);
+    let land: (v: unknown) => void = () => {};
+    vi.spyOn(api, "saveProgress").mockReturnValue(new Promise((r) => (land = r)) as never);
+
+    const { unmount } = mountPlayer(qc, new Map());
+    await settle();
+    unmount();
+    const flushes = progressFlushes();
+    expect(flushes).toHaveLength(1);
+    land({ video_id: VID, position_seconds: 830 });
+    await flushes[0];
+    expect(cachedPos(qc)).toBe(830);
+    await settle();
+    expect(progressFlushes()).toHaveLength(0);
+  });
+
   it("mirrors what the SERVER stored, not what the player read", async () => {
     const qc = new QueryClient();
     seedFeed(qc, video({ position_seconds: 748 }));

frontend/src/lib/playerModalHooks.ts

@@ -19,7 +19,7 @@ import {
 import { useBackToExit } from "./history";
 import { api, type Video } from "./api";
 import { qk } from "./queryKeys";
-import { patchCachedProgress } from "./progressCache";
+import { patchCachedProgress, trackProgressFlush } from "./progressCache";
 import type { YTNamespace, YTPlayer, YTPlayerEvent } from "./youtube";
 import { logYouTubeApiFailed, logYouTubeError, logYouTubeState } from "./mediaLog";
 import type { AutoMode, LoopMode } from "./playerModes";
@@ -527,15 +527,18 @@ export function useYtProgress({
       window.removeEventListener("pagehide", onHide);
       // Final flush, then mark the feed + playlists stale so resume bars reflect this session on the next
       // natural refetch — WITHOUT refetching every loaded feed page on each player close (PERF-2).
-      void Promise.resolve(persist()).finally(() => {
-        // Write this session's stored positions INTO the cached lists first. Marking them stale is
-        // not enough on its own: nothing refetches the feed until it remounts (staleTime 30s,
-        // refetchOnWindowFocus off), so a reopen used to resume from the position the cache was
-        // fetched with — and the next checkpoint then persisted that older value over this one.
-        patchCachedProgress(qc, sessionPos);
-        qc.invalidateQueries({ queryKey: qk.feed(), refetchType: "none" });
-        qc.invalidateQueries({ queryKey: qk.playlist(), refetchType: "none" });
-      });
+      // Tracked, so a list refetch on the player's close waits for it (`progressFlushes`).
+      trackProgressFlush(
+        Promise.resolve(persist()).finally(() => {
+          // Write this session's stored positions INTO the cached lists first. Marking them stale is
+          // not enough on its own: nothing refetches the feed until it remounts (staleTime 30s,
+          // refetchOnWindowFocus off), so a reopen used to resume from the position the cache was
+          // fetched with — and the next checkpoint then persisted that older value over this one.
+          patchCachedProgress(qc, sessionPos);
+          qc.invalidateQueries({ queryKey: qk.feed(), refetchType: "none" });
+          qc.invalidateQueries({ queryKey: qk.playlist(), refetchType: "none" });
+        }),
+      );
       const p = playerRef.current;
       if (p && typeof p.destroy === "function") {
         try {

frontend/src/lib/progressCache.ts

@@ -25,6 +25,19 @@ const isItemList = (v: unknown): v is ItemList =>
  * read: the endpoint clears trivially-early and near-finished checkpoints to 0, and mirroring our
  * own reading would re-introduce that disagreement on the client.
  */
+// The player's closing flushes still on their way: its last checkpoint and the patch above. A list
+// refetched before one lands read the older checkpoint and, landing after the patch, put it back.
+const flushes = new Set<Promise<unknown>>();
+
+/** Track a closing flush until it settles (`progressFlushes` hands out the ones still running). */
+export function trackProgressFlush(flush: Promise<unknown>): void {
+  flushes.add(flush);
+  const settled = () => void flushes.delete(flush);
+  flush.then(settled, settled);
+}
+
+export const progressFlushes = (): Promise<unknown>[] => [...flushes];
+
 export function patchCachedProgress(qc: QueryClient, positions: Map<string, number>): void {
   if (positions.size === 0) return;