fix(plex): subtitles draw only the player's own track, lifted in the player's own pixels

A direct-played file's own subtitle stream (WebKit lists an mp4's mov_text) was switched to hidden and drawn under any choice, Off included: only the player's <track> is kept hidden and read, other subtitle tracks are switched off. The lift was measured in screen pixels and applied inside the x1.25 windowed player, and re-measured off a box carrying its own (or a mid-slide) transform: an untransformed outer box is measured, the distance divided by the scale, and a cue that just appeared is placed, not slid from where the last one was left.

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

@@ -440,9 +440,10 @@ 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", () => {
-  /** A <video> whose text-track list fires `addtrack` / `change` like a browser's. */
+  /** A <video> whose text-track list fires `addtrack` / `change` like a browser's. Each mode is a
+   *  track of the player's own <track>; `mediaAdds` adds one from the media itself instead. */
   function videoWithTracks(...modes: TextTrackMode[]) {
-    const list: { mode: TextTrackMode }[] = modes.map((mode) => ({ mode }));
+    const list: { kind: string; mode: TextTrackMode }[] = [];
     const tracks = Object.assign(new EventTarget(), {
       get length() {
         return list.length;
@@ -451,10 +452,20 @@ describe("usePlexTracks — who draws the cues", () => {
     });
     const video = document.createElement("video");
     Object.defineProperty(video, "textTracks", { value: tracks });
+    const add = (mode: TextTrackMode, own: boolean) => {
+      const track = { kind: "subtitles", mode };
+      if (own) {
+        const el = document.createElement("track");
+        Object.defineProperty(el, "track", { value: track });
+        video.appendChild(el);
+      }
+      list.push(track);
+    };
+    for (const mode of modes) add(mode, true);
     // What a browser does with a freshly mounted `default` <track>: it switches it on by itself.
     const browserAddsShowing = () =>
       act(() => {
-        list.push({ mode: "showing" });
+        add("showing", true);
         tracks.dispatchEvent(new Event("addtrack"));
       });
     const browserShows = () =>
@@ -462,7 +473,13 @@ describe("usePlexTracks — who draws the cues", () => {
         list[0]!.mode = "showing";
         tracks.dispatchEvent(new Event("change"));
       });
-    return { video, list, browserAddsShowing, browserShows };
+    // A direct-played file's own subtitle stream, which WebKit lists — and may switch on.
+    const mediaAdds = (mode: TextTrackMode) =>
+      act(() => {
+        add(mode, false);
+        tracks.dispatchEvent(new Event("addtrack"));
+      });
+    return { video, list, browserAddsShowing, browserShows, mediaAdds };
   }
 
   function mountOn(video: HTMLVideoElement) {
@@ -504,4 +521,13 @@ describe("usePlexTracks — who draws the cues", () => {
     expect(list[0]!.mode).toBe("hidden");
     unmount();
   });
+
+  it("switches off a subtitle track that is not the player's own, and leaves its own hidden", () => {
+    const { video, list, mediaAdds } = videoWithTracks("disabled");
+    const { unmount } = mountOn(video);
+    mediaAdds("hidden");
+    mediaAdds("showing");
+    expect(list.map((t) => t.mode)).toEqual(["hidden", "disabled", "disabled"]);
+    unmount();
+  });
 });