fix(plex): the subtitle poll backs off on errors, a failed <track> is replaced, and a resume asks for the right window

Deep-review R2, plus one defect found while verifying it on localdev.

- The status poll re-armed every second after a FAILED request: `data` stays empty on an error,
  so it read as "still preparing" for as long as the player was open — and the request was not
  quiet, so a persistent 500 re-opened the global error dialog every second. It now backs off to
  10 s (a blip still heals on its own) and is quiet; the player already shows the failure inline.
- A <track> whose own VTT fetch failed never fetched again: with the server's answer unchanged its
  key was unchanged, so React kept the dead element — silently blank subtitles. The key (now built
  in the hook, where it is tested) carries a failed-load count; after two remounts the track is
  reported as unavailable instead.
- Found in UAT: until the first timeupdate `abs` is 0, so after a resume every poll said at=0 and
  the first window was cut around the head of the film (a resume at 20:00 polled at=0 for as long
  as it stayed paused). The poll reports the session's start until playback ticks. And the window's
  dedup identity is the track plus a window-length bucket of its start — keyed on the track alone
  (R1), the right window queued behind that wrong one.

The poll's start/stop rules finally have tests (every earlier one resolved a single fixed answer).
Verified on localdev against Westworld S1E6 resumed at 20:00: the polls carried at=1197, one
subtitle line on screen at 21:02, the poll stopped at `ready`; the real file's window (56 cues) +
full pass (608) merged to 608 — the old merge would have served 664. 11 mutations, all red.

backend/app/plex/subtitles.py

@@ -439,14 +439,17 @@ def _schedule(rating_key: str, sub_ord: int, at: float, want_window: bool) -> No
     full pass that makes it complete. Each is deduplicated by identity, so a 1 s status poll (or a
     second viewer) joins the running job instead of starting another ffmpeg.
 
-    A window's identity is its TRACK, never its position. The poll sends the live playhead, so a
+    A window's identity is its track plus WHICH STRETCH of the film it is for — never the exact
-    position-keyed window minted a new identity every second and queued one more read of the file per
+    position. The poll sends the live playhead, so a position-keyed window minted a new identity every
-    poll until the first window landed. One window per track is in flight at a time; a seek past it
+    second and queued one more read of the file per poll until the first window landed; bucketing the
-    is picked up by the next poll after it publishes. The full pass is per ITEM, because one read
+    start by the window's own length makes a playhead creeping along join the job already running. It
-    fans out to every embedded text track of the file."""
+    must not be the track alone either: then a window cut around the wrong spot (the first poll after
+    a resume can still report 0) made the right one wait for it. The full pass is per ITEM, because
+    one read fans out to every embedded text track of the file."""
     jobs: list[tuple[tuple, float | None]] = []
     if want_window:
-        jobs.append((("win", rating_key, sub_ord), max(0.0, at - WINDOW_PRE_S)))
+        w_start = max(0.0, at - WINDOW_PRE_S)
+        jobs.append((("win", rating_key, sub_ord, int(w_start // WINDOW_LEN_S)), w_start))
     jobs.append((("full", rating_key), None))
     for ident, window_start in jobs:
         with _LOCK:

backend/tests/test_plex_subtitle_prepare.py

@@ -395,6 +395,17 @@ def test_polls_that_follow_the_playhead_join_one_window(monkeypatch):
     assert len(pool.jobs) == 2, "one window + the one full pass"
 
 
+def test_a_window_for_a_different_stretch_does_not_wait_behind_the_first(monkeypatch):
+    """Measured on localdev: after a resume the first poll can still report 0 (the position is only
+    known once playback ticks), so the first window is cut around the wrong spot. The poll that
+    follows, at the real position, must get its own window at once — not queue behind that one."""
+    pool = _ManualPool()
+    monkeypatch.setattr(subtitles, "_POOL", pool)
+    subtitles.ensure("98614", 1, at=0.0)
+    subtitles.ensure("98614", 1, at=1198.0)
+    assert sorted(a[3] for _, a in pool.jobs if a[3] is not None) == [0.0, 1168.0]
+
+
 def test_a_queued_window_that_was_covered_meanwhile_reads_nothing(monkeypatch):
     """Every embedded track rides each pass over its file, so by the time a queued window for one
     language runs, another language's window may already have published it."""

frontend/src/components/PlexPlayer.tsx

@@ -380,6 +380,7 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
     subShowable,
     subFail,
     subGen,
+    subTrackKey,
     onSubTrackError,
   } = usePlexTracks({
     id,
@@ -873,10 +874,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
             above flips modes; the browser fetches a track's VTT the first time it's shown. */}
         {selectedSub && subShowable && (
           <track
-            // One track = the selected sub only (no multi-track overlap). Key by the shifted-offset so
+            // One track = the selected sub only (no multi-track overlap). The key (see usePlexTracks)
-            // a seek-restart or a (debounced) sub-offset change remounts → fresh VTT fetch, and by the
+            // is what remounts it for a fresh VTT fetch: a seek-restart, an offset change, a widened
-            // extraction generation so a widened track replaces the first short window.
+            // track, or a failed load.
-            key={`${id}-${selectedSub.ord}-${subTrackOffset.toFixed(2)}-${subGen}`}
+            key={subTrackKey}
             kind="subtitles"
             src={api.plexSubtitleUrl(id, selectedSub.ord, subTrackOffset, subGen)}
             srcLang={selectedSub.language ?? undefined}

frontend/src/lib/api/plex.ts

@@ -672,8 +672,11 @@ export const plexApi = {
     code: string | null;
     gen: number;
   }> =>
+    // `quiet`: the player reports a failed poll inline, so no global error dialog on top of it.
     req(
       `/api/plex/subtitle/${encodeURIComponent(id)}/${ord}/status?at=${Math.max(0, Math.floor(at))}`,
+      undefined,
+      { quiet: true },
     ),
   // `final` marks a settled checkpoint (pause / page-hide / leaving the player) as opposed to the
   // periodic 10s tick — the backend mirrors the resume position to a linked Plex account only on a

frontend/src/lib/plexPlayerHooks.subtitle.dom.test.tsx

@@ -9,12 +9,13 @@
 // until the cache happened to be warm. The extraction now runs in the background, publishing a
 // window around the playhead first, and this hook's poll is what decides when there is something to
 // mount and what to say while there isn't.
-import { describe, it, expect, vi, afterEach } from "vitest";
+import { describe, it, expect, vi, afterEach, beforeEach } from "vitest";
-import { cleanup, renderHook, waitFor } from "@testing-library/react";
+import { act, cleanup, renderHook, waitFor } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { createRef, type ReactNode, type MutableRefObject } from "react";
 import { usePlexTracks } from "./plexPlayerHooks";
 import { api, type PlexItemDetail } from "./api";
+import { dismissError, getError } from "./errorDialog";
 import type Hls from "hls.js";
 
 const RK = "98614";
@@ -31,7 +32,7 @@ const detail = (): PlexItemDetail =>
     markers: [],
   }) as unknown as PlexItemDetail;
 
-function mount(qc: QueryClient) {
+function mount(qc: QueryClient, { sessionK = 0, abs = 3600 } = {}) {
   const wrapper = ({ children }: { children: ReactNode }) => (
     <QueryClientProvider client={qc}>{children}</QueryClientProvider>
   );
@@ -40,7 +41,7 @@ function mount(qc: QueryClient) {
       usePlexTracks({
         id: RK,
         detail: detail(),
-        sessionK: 0,
+        sessionK,
         subOffset: 0,
         subPos: 88,
         loadSession: async () => {},
@@ -50,7 +51,7 @@ function mount(qc: QueryClient) {
         multiAudioRef: { current: false },
         audioRef: { current: 0 } as MutableRefObject<number | null>,
         aoffRef: { current: 0 },
-        absRef: { current: 3600 },
+        absRef: { current: abs },
       }),
     { wrapper },
   );
@@ -91,6 +92,18 @@ describe("usePlexTracks — the <track> waits for the extraction", () => {
     await waitFor(() => expect(status).toHaveBeenCalledWith(RK, 1, 3600));
   });
 
+  it("sends the session's start before playback has ticked, not the head of the film", async () => {
+    // A resumed session starts minutes in, but `abs` stays 0 until the first timeupdate — and every
+    // poll used to say at=0, so the first window was cut around the head of the film (measured on
+    // localdev: a resume at 20:00 polled at=0 for as long as it stayed paused).
+    const status = vi
+      .spyOn(api, "plexSubtitleStatus")
+      .mockResolvedValue({ state: "preparing", code: null, gen: 0 });
+    const { result } = mount(client(), { sessionK: 1198, abs: 0 });
+    result.current.setSubOrd(1);
+    await waitFor(() => expect(status).toHaveBeenCalledWith(RK, 1, 1198));
+  });
+
   it("becomes showable on a PARTIAL track — a window around the playhead is what is needed now", async () => {
     vi.spyOn(api, "plexSubtitleStatus").mockResolvedValue({
       state: "partial",
@@ -138,3 +151,93 @@ describe("usePlexTracks — the <track> waits for the extraction", () => {
     expect(result.current.subShowable).toBe(false);
   });
 });
+
+// The poll's refetch rules ARE the feature: nothing else turns "preparing" into subtitles on screen,
+// and nothing else stops a settled track from being asked about for the length of the film. Every
+// test above resolves one fixed answer, so none of them ever reached the refetch callback.
+describe("usePlexTracks — when the status poll asks again, and when it stops", () => {
+  beforeEach(() => {
+    vi.useFakeTimers({ shouldAdvanceTime: true });
+  });
+  afterEach(() => {
+    cleanup();
+    vi.useRealTimers();
+    vi.restoreAllMocks();
+  });
+
+  it("keeps asking until a window lands, then shows it without the viewer doing anything", async () => {
+    const status = vi
+      .spyOn(api, "plexSubtitleStatus")
+      .mockResolvedValueOnce({ state: "preparing", code: null, gen: 0 })
+      .mockResolvedValue({ state: "partial", code: null, gen: 7 });
+    const { result } = mount(client());
+    result.current.setSubOrd(0);
+    await waitFor(() => expect(status).toHaveBeenCalledTimes(1));
+    expect(result.current.subShowable).toBe(false);
+    await vi.advanceTimersByTimeAsync(1100);
+    await waitFor(() => expect(result.current.subShowable).toBe(true));
+  });
+
+  it.each([
+    ["the whole track is there", { state: "ready" as const, code: null, gen: 3 }],
+    ["it can never be shown", { state: "failed" as const, code: "bitmap", gen: 0 }],
+  ])("stops asking once the answer is settled — %s", async (_, answer) => {
+    const status = vi.spyOn(api, "plexSubtitleStatus").mockResolvedValue(answer);
+    const { result } = mount(client());
+    result.current.setSubOrd(0);
+    await waitFor(() => expect(status).toHaveBeenCalledTimes(1));
+    await vi.advanceTimersByTimeAsync(5000);
+    expect(status).toHaveBeenCalledTimes(1);
+  });
+
+  it("backs off after a failed request, and still asks again so a blip heals on its own", async () => {
+    // A failed request leaves `data` empty; it used to read as "still preparing" and re-ask every
+    // second for as long as the player stayed open.
+    const status = vi.spyOn(api, "plexSubtitleStatus").mockRejectedValue(new Error("offline"));
+    const { result } = mount(client());
+    result.current.setSubOrd(0);
+    await waitFor(() => expect(result.current.subFail).toBe("failed"));
+    expect(status).toHaveBeenCalledTimes(1);
+    await vi.advanceTimersByTimeAsync(5000);
+    expect(status).toHaveBeenCalledTimes(1);
+    await vi.advanceTimersByTimeAsync(6000);
+    expect(status).toHaveBeenCalledTimes(2);
+  });
+
+  it("does not raise the global error dialog when it fails — the player says so inline", async () => {
+    dismissError();
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response(JSON.stringify({ detail: "boom" }), {
+        status: 500,
+        headers: { "content-type": "application/json" },
+      }),
+    );
+    await expect(api.plexSubtitleStatus(RK, 0, 0)).rejects.toThrow();
+    expect(getError()).toBeNull();
+  });
+});
+
+describe("usePlexTracks — a <track> whose own VTT fetch failed", () => {
+  afterEach(() => {
+    cleanup();
+    vi.restoreAllMocks();
+  });
+
+  it("is replaced by a fresh element a couple of times, then reported instead of left blank", async () => {
+    // A failed <track> never fetches again, and while the server's answer is unchanged its key was
+    // unchanged too — so React kept the dead element and the subtitles stayed silently blank.
+    vi.spyOn(api, "plexSubtitleStatus").mockResolvedValue({ state: "partial", code: null, gen: 5 });
+    const { result } = mount(client());
+    result.current.setSubOrd(0);
+    await waitFor(() => expect(result.current.subShowable).toBe(true));
+    for (let i = 0; i < 2; i++) {
+      const before = result.current.subTrackKey;
+      act(() => result.current.onSubTrackError());
+      await waitFor(() => expect(result.current.subTrackKey).not.toBe(before));
+      expect(result.current.subShowable).toBe(true);
+    }
+    act(() => result.current.onSubTrackError());
+    await waitFor(() => expect(result.current.subShowable).toBe(false));
+    expect(result.current.subFail).toBe("failed");
+  });
+});

frontend/src/lib/plexPlayerHooks.ts

@@ -349,6 +349,11 @@ export function useAutoSkip(opts: {
  *  fold happens here rather than multiplying near-identical translations. */
 export type SubFailReason = "bitmap" | "missing" | "failed";
 
+// The subtitle status poll's cadence after a FAILED request (see usePlexTracks), and how many times a
+// <track> whose own VTT fetch failed is remounted before the track is reported as unavailable.
+const SUB_POLL_ERROR_MS = 10_000;
+const SUB_TRACK_RETRIES = 2;
+
 function subFailReason(code: string | null | undefined): SubFailReason {
   if (code === "bitmap") return "bitmap";
   if (code === "no_item" || code === "no_track" || code === "no_file" || code === "not_fetchable")
@@ -394,6 +399,11 @@ export function usePlexTracks(opts: {
   /** The cached VTT's generation — it grows as the background extraction widens the track, so it
    *  belongs in the <track> key/URL or the browser keeps serving the first, short window. */
   subGen: number;
+  /** The <track>'s React key. Everything that must produce a FRESH element — and so a fresh VTT
+   *  fetch — is in it: the item and track, the shifted offset (a seek-restart or a debounced offset
+   *  change), the extraction generation (a widened track replaces the first short window), and the
+   *  failed-load count (a <track> whose fetch failed is replaced rather than left dead). */
+  subTrackKey: string;
   onSubTrackError: () => void;
 } {
   const { id, detail, sessionK, subOffset, subPos, loadSession, patchPrefs } = opts;
@@ -486,14 +496,28 @@ export function usePlexTracks(opts: {
   // around the playhead first, and this poll is what tells us when there is something to mount.
   const subStatusQ = useQuery({
     queryKey: qk.plexSubtitle(id, subOrd ?? -1),
-    queryFn: () => api.plexSubtitleStatus(id, subOrd as number, absRef.current ?? 0),
+    // The position is only known once playback has ticked: until the first timeupdate `abs` is 0
+    // even though a resumed session starts minutes in — and every poll said `at=0`, so the first
+    // window was cut around the head of the film instead of where the viewer is (measured on
+    // localdev: a resume at 20:00 polled `at=0` for as long as it stayed paused). The session's own
+    // start is the best answer until then.
+    queryFn: () =>
+      api.plexSubtitleStatus(id, subOrd as number, absRef.current > 0 ? absRef.current : sessionK),
     enabled: subOrd != null,
     // Keep polling until the track is COMPLETE, not just showable: each poll carries the current
     // position, so seeking past the extracted window is what asks for the next one. A settled
     // answer — the whole track, or a reason it can never be shown — stops the poll; without that
     // stop, a bitmap track would poll for the length of the film AND keep re-arming the extraction
     // every time the server's short failure memo expired.
+    //
+    // A FAILED request is neither: it says nothing about the track. It must not keep the 1 s cadence
+    // (it used to — `data` stays empty on an error, so every failure read as "still preparing" and
+    // re-armed a second later, for as long as the player was open), but it must not end the poll
+    // either, or one dropped request would leave "unavailable" on screen for a track that is fine.
+    // So it backs off to SUB_POLL_ERROR_MS; the player shows the failure inline meanwhile, which is
+    // also why the request is `quiet` (no global error dialog re-opening under the film).
     refetchInterval: (q) => {
+      if (q.state.status === "error") return SUB_POLL_ERROR_MS;
       const st = q.state.data?.state;
       if (st === "ready" || st === "failed") return false;
       return st === "partial" ? 2000 : 1000;
@@ -502,20 +526,32 @@ export function usePlexTracks(opts: {
     retry: false,
   });
   const subStatus = subOrd == null ? null : (subStatusQ.data ?? null);
+  // A <track> whose own VTT fetch failed never fetches again, and re-asking the status cannot fix
+  // that on its own: while the answer (and its `gen`) is unchanged the key is unchanged, so React
+  // keeps the dead element and the subtitles stay silently blank. So a failed load bumps a counter
+  // that is part of the key — a fresh element, a fresh fetch — and after SUB_TRACK_RETRIES the track
+  // is reported as unavailable instead of left blank. The count belongs to one track generation: a
+  // different track, item or `gen` starts from zero.
+  const loadKey = `${id}:${subOrd}:${subStatus?.gen ?? 0}`;
+  const [loadFails, setLoadFails] = useState({ key: "", n: 0 });
+  const subRetry = loadFails.key === loadKey ? loadFails.n : 0;
+  const subGaveUp = subRetry > SUB_TRACK_RETRIES;
   // Mount the <track> for a `partial` track too: a window around the playhead is exactly what the
   // viewer needs right now, and the poll widens it underneath them.
-  const subShowable = subStatus?.state === "ready" || subStatus?.state === "partial";
+  const subShowable =
+    (subStatus?.state === "ready" || subStatus?.state === "partial") && !subGaveUp;
   const subFail: SubFailReason | null =
-    subStatusQ.isError && subOrd != null
+    (subStatusQ.isError || subGaveUp) && subOrd != null
       ? "failed"
       : subStatus?.state === "failed"
         ? subFailReason(subStatus.code)
         : null;
-  // A <track> that errors anyway (the cached VTT was evicted between the poll and the fetch) goes
-  // back to polling rather than staying silently blank.
   const onSubTrackError = useCallback(() => {
+    setLoadFails((f) => ({ key: loadKey, n: (f.key === loadKey ? f.n : 0) + 1 }));
+    // …and ask again: if the cached VTT was evicted between the poll and the fetch, the answer drops
+    // back to `preparing` and the poll takes over until it has been re-extracted.
     if (subOrd != null) qc.invalidateQueries({ queryKey: qk.plexSubtitle(id, subOrd) });
-  }, [qc, id, subOrd]);
+  }, [qc, id, subOrd, loadKey]);
 
   // Text subtitles offered in the picker; image subs (PGS/VobSub) can't be shown as text.
   const textSubs = (detail?.subtitle_streams ?? []).filter((s) => s.text);
@@ -586,6 +622,7 @@ export function usePlexTracks(opts: {
     subShowable,
     subFail,
     subGen: subStatus?.gen ?? 0,
+    subTrackKey: `${id}-${subOrd}-${subTrackOffset.toFixed(2)}-${subStatus?.gen ?? 0}-${subRetry}`,
     onSubTrackError,
   };
 }