fix(plex): subtitles survive hls.js and a remounted track

hls.js emptied the cues of every text track on the element when a manifest
loaded or the media attached, our own <track> included, so a VTT that loaded
first showed nothing for the session; its subtitle and caption controllers are
no longer built. A remounted default <track> was switched to showing by the
browser after the hide had run, drawing the cues natively under ours; the hide
now follows every added track and mode change.

frontend/src/lib/hlsPort.test.ts

@@ -0,0 +1,42 @@
+// hls.js empties the cues of every text track on the <video> when a manifest loads or the media
+// attaches — the Plex player's own subtitle <track> included, so a VTT that loaded first showed
+// nothing for the rest of the session. Its subtitle and caption controllers are not built at all.
+import { describe, expect, it, vi } from "vitest";
+
+const built: Record<string, unknown>[] = [];
+vi.mock("hls.js", () => {
+  class FakeHls {
+    static isSupported = () => true;
+    static Events = {};
+    static ErrorTypes = {};
+    constructor(config: Record<string, unknown>) {
+      built.push(config);
+    }
+    on() {}
+  }
+  return { default: FakeHls };
+});
+
+const { createHlsPort } = await import("./hlsPort");
+
+describe("createHlsPort", () => {
+  it("builds hls.js without the controllers that clear every text track's cues", () => {
+    const noop = () => {};
+    createHlsPort({
+      onManifestParsed: noop,
+      onAudioTracksUpdated: noop,
+      onFragLoaded: noop,
+      onError: noop,
+    });
+    const config = built[0] ?? {};
+    for (const key of [
+      "subtitleTrackController",
+      "subtitleStreamController",
+      "timelineController",
+    ]) {
+      // Set, as undefined: hls.js spreads the config over its defaults, so an absent key keeps them.
+      expect(key in config).toBe(true);
+      expect(config[key]).toBeUndefined();
+    }
+  });
+});

frontend/src/lib/hlsPort.ts

@@ -12,7 +12,19 @@ export function createHlsPort(handlers: HlsHandlers): HlsPort | null {
   // up with the live-edge timeline → 0 fragments load → a permanent stall. But the seek-restart model
   // means fragment 0 IS the requested content position (`?t=` positioned the server ffmpeg there),
   // so it always starts VOD-style at 0 and buffers forward up to maxBufferLength.
-  const hls = new Hls({ maxBufferLength: 30, enableWorker: true, startPosition: 0 });
+  //
+  // No subtitle or caption controllers (a controller left unset is never built): the subtitles are
+  // a <track> of ours, out of band, and hls.js empties the cues of EVERY text track on the element
+  // when a manifest loads or the media attaches/detaches — ours included, so a VTT that loaded before
+  // the manifest showed nothing for the rest of the session.
+  const hls = new Hls({
+    maxBufferLength: 30,
+    enableWorker: true,
+    startPosition: 0,
+    subtitleTrackController: undefined,
+    subtitleStreamController: undefined,
+    timelineController: undefined,
+  });
   hls.on(Hls.Events.MANIFEST_PARSED, () => handlers.onManifestParsed());
   hls.on(Hls.Events.AUDIO_TRACKS_UPDATED, () => handlers.onAudioTracksUpdated());
   // The delivered bitrate is measured per fragment: the master playlist's own BANDWIDTH is not a

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

@@ -440,14 +440,36 @@ describe("usePlexTracks — the debounced A/V-offset restart", () => {
 // The browser no longer draws the cues: its rendering sat under the control bar on an iPad, and a cue
 // placed by `line` cannot move out of the way. The track stays loaded, as `hidden`, for PlexSubtitles.
 describe("usePlexTracks — who draws the cues", () => {
-  it("keeps the mounted track hidden: loaded and tracked, not drawn by the browser", () => {
-    const track = { mode: "disabled" as TextTrackMode };
+  /** A <video> whose text-track list fires `addtrack` / `change` like a browser's. */
+  function videoWithTracks(...modes: TextTrackMode[]) {
+    const list: { mode: TextTrackMode }[] = modes.map((mode) => ({ mode }));
+    const tracks = Object.assign(new EventTarget(), {
+      get length() {
+        return list.length;
+      },
+      [Symbol.iterator]: () => list[Symbol.iterator](),
+    });
     const video = document.createElement("video");
-    Object.defineProperty(video, "textTracks", { value: [track] });
+    Object.defineProperty(video, "textTracks", { value: tracks });
+    // What a browser does with a freshly mounted `default` <track>: it switches it on by itself.
+    const browserAddsShowing = () =>
+      act(() => {
+        list.push({ mode: "showing" });
+        tracks.dispatchEvent(new Event("addtrack"));
+      });
+    const browserShows = () =>
+      act(() => {
+        list[0]!.mode = "showing";
+        tracks.dispatchEvent(new Event("change"));
+      });
+    return { video, list, browserAddsShowing, browserShows };
+  }
+
+  function mountOn(video: HTMLVideoElement) {
     const wrapper = ({ children }: { children: ReactNode }) => (
       <QueryClientProvider client={client()}>{children}</QueryClientProvider>
     );
-    const { unmount } = renderHook(
+    return renderHook(
       () =>
         usePlexTracks({
           id: RK,
@@ -464,7 +486,22 @@ describe("usePlexTracks — who draws the cues", () => {
         }),
       { wrapper },
     );
-    expect(track.mode).toBe("hidden");
+  }
+
+  it("keeps the mounted track hidden: loaded and tracked, not drawn by the browser", () => {
+    const { video, list } = videoWithTracks("disabled");
+    const { unmount } = mountOn(video);
+    expect(list[0]!.mode).toBe("hidden");
+    unmount();
+  });
+
+  it("hides a remounted track the browser switched on by itself", () => {
+    const { video, list, browserAddsShowing, browserShows } = videoWithTracks("disabled");
+    const { unmount } = mountOn(video);
+    browserAddsShowing();
+    expect(list.map((t) => t.mode)).toEqual(["hidden", "hidden"]);
+    browserShows();
+    expect(list[0]!.mode).toBe("hidden");
     unmount();
   });
 });

frontend/src/lib/plexPlayerHooks.ts

@@ -852,14 +852,25 @@ export function usePlexTracks(opts: {
   // sessionK is already lag-compensated (see HlsEngine), so it is the true content-zero the VTT
   // must be shifted by; the user offset nudges on top.
   const subTrackOffset = sessionK - subOffApplied;
-  // Force the single mounted track to "hidden": loaded and its cues tracked (its VTT loads on mount;
-  // `default` alone can be flaky after a remount), but drawn by PlexSubtitles, not by the browser.
-  // Re-runs when the selection or the shifted-track identity changes (fresh <track>).
+  // Keep every track "hidden": loaded and its cues tracked, but drawn by PlexSubtitles, not by the
+  // browser. Not once per selection: the <track> is remounted on its own key (a grown window, a
+  // retry), and the browser switches a fresh `default` track to "showing" a task later — which drew
+  // the cues a second time, natively, under ours. So it is re-applied on every track the list gains
+  // and on every mode change the browser makes.
   useEffect(() => {
-    const video = videoRef.current;
-    if (!video) return;
-    for (const tr of Array.from(video.textTracks)) tr.mode = "hidden";
-  }, [subOrd, id, subTrackOffset, videoRef]);
+    const tracks = videoRef.current?.textTracks;
+    if (!tracks) return;
+    const hide = () => {
+      for (const tr of Array.from(tracks)) if (tr.mode !== "hidden") tr.mode = "hidden";
+    };
+    hide();
+    tracks.addEventListener("addtrack", hide);
+    tracks.addEventListener("change", hide);
+    return () => {
+      tracks.removeEventListener("addtrack", hide);
+      tracks.removeEventListener("change", hide);
+    };
+  }, [subOrd, id, videoRef]);
 
   return {
     audioOrd,