Keep a playing video when the window comes back to the front (#402)

Chrome draws no frames for a hidden page, so the scene stopped asking
for its videos. The sweeper of the VideoLibrary kept running and
dropped every player after 2 s. Back in the window, the video started
over, and Pixi warned that a texture source was destroyed while still
bound to a shader.

- The sweeper skips a hidden page. The time hidden does not count as
  idle, the 2 s start once the page is shown again.
- A dropped player only unloads its video source instead of destroying
  it. A shader that still holds the source no longer warns.
- CI serves videos through a new router for php -S. The built-in
  server answers no Range requests, so Chrome could not seek. e2e 66
  had 4 red checks in the suite job of d48f174 because of it. nginx on
  stein2 hid this.

Tests:
- Unit video-library.test.ts (new): 3 tests
- e2e 66 (atx): ALL PASSED, 18 checks
- Router checked with curl: 206 with the right bytes
- Counter-check: the old sweep fails both hidden-page tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/viewport/VideoLibrary.ts

@@ -36,6 +36,15 @@ export class VideoLibrary {
   private heard = false;
   private paused = false;
   private sweeper: ReturnType<typeof setInterval> | null = null;
+  /** When the page was last shown, by `performance.now()`; see `sweep`. */
+  private shown = 0;
+
+  constructor() {
+    if (typeof document === 'undefined') return;
+    document.addEventListener('visibilitychange', () => {
+      if (!document.hidden) this.shown = performance.now();
+    });
+  }
 
   /** Runs `fn` whenever a player has its first picture; returns the unsubscribe. */
   onChange(fn: () => void): () => void {
@@ -85,12 +94,16 @@ export class VideoLibrary {
     this.stopSweeping();
   }
 
-  /** Drops the players nobody asked for lately; a paused run asks for nothing and keeps them. */
+  /**
+   * Drops the players nobody asked for lately; a paused run asks for nothing and keeps them. So
+   * does a hidden page: Chrome draws no frames for it while its timers go on, and a sprite that
+   * still wears a video would lose it. The time hidden does not count as idle.
+   */
   private sweep(): void {
-    if (this.paused) return;
+    if (this.paused || document.hidden) return;
     const now = performance.now();
     for (const [key, player] of this.players)
-      if (now - player.asked > IDLE_MS) {
+      if (now - Math.max(player.asked, this.shown) > IDLE_MS) {
         this.drop(player);
         this.players.delete(key);
       }
@@ -181,12 +194,14 @@ export class VideoLibrary {
     player.element.load();
     // The texture itself stays whole: a wearer that has not synced since still draws it, and a
     // destroyed one has no source and threw in the batcher. It shows nothing until the wearer asks
-    // again; only the video behind it is let go.
+    // again. The video source is only unloaded, not destroyed: a shader may still hold it until
+    // its wearer syncs, and Pixi warns about every destroyed source still bound to one.
     const texture = player.texture;
     if (!texture) return;
-    const video = texture.source;
+    const video = texture.source as VideoSource;
     texture.source = Texture.EMPTY.source;
-    video.destroy();
+    video.autoUpdate = false;
+    video.unload();
   }
 
   private notify(): void {