fix: Emit ratechange event when playbackRate stays at 0 (#10130)

This can happen when using playbackRate values ​​that are outside the
range supported by the platform, and we have to use our internal
alternative. See the implementation at
https://github.com/shaka-project/shaka-player/blob/main/lib/media/play_rate_controller.js

lib/player.js

@@ -5275,6 +5275,9 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       return;
     }
 
+    const prevRealRate = this.video_.playbackRate;
+    const prevRate = this.playRateController_.getRealRate();
+
     this.playRateController_.set(rate);
 
     if (this.loadMode_ == shaka.Player.LoadMode.MEDIA_SOURCE) {
@@ -5282,6 +5285,21 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       this.useTrickPlayTrackIfAvailable(useTrickPlayTrack && rate != 1);
     }
     this.setupTrickPlayEventListeners_(rate);
+
+    // When trick play falls back to manual seeking (for example, rewind or
+    // unsupported playback rates), PlayRateController keeps the media element
+    // playbackRate at 0 and advances the playhead with a timer instead.
+    //
+    // Because the underlying media element playbackRate remains unchanged,
+    // the browser will not fire a native 'ratechange' event in this case.
+    //
+    // Invoke onRateChange_() manually so the player can run the normal
+    // playback-rate update flow and dispatch the corresponding RateChange
+    // event for the effective trick play rate change.
+    if (prevRealRate === 0 && this.video_.playbackRate === 0 &&
+        rate != prevRate) {
+      this.onRateChange_();
+    }
   }
 
   /**

test/player_integration.js

@@ -351,6 +351,46 @@ describe('Player', () => {
       expect(video.playbackRate).toBe(1);
     });
 
+    it('dispatch ratechange event', async () => {
+      await player.load('test:sintel_compiled');
+      await video.play();
+      await waiter.waitUntilPlayheadReachesOrFailOnTimeout(video, 1, 10);
+
+      video.pause();
+
+      let numberOfRatechangeEvents = 0;
+      eventManager.listen(player, 'ratechange', () => {
+        numberOfRatechangeEvents++;
+      });
+
+      player.trickPlay(2);
+      expect(player.getPlaybackRate()).toBe(2);
+
+      await shaka.test.Util.shortDelay();
+
+      player.trickPlay(32);
+      expect(player.getPlaybackRate()).toBe(32);
+
+      await shaka.test.Util.shortDelay();
+
+      player.trickPlay(64);
+      expect(player.getPlaybackRate()).toBe(64);
+
+      await shaka.test.Util.shortDelay();
+
+      player.trickPlay(-1);
+      expect(player.getPlaybackRate()).toBe(-1);
+
+      await shaka.test.Util.shortDelay();
+
+      player.cancelTrickPlay();
+      expect(player.getPlaybackRate()).toBe(1);
+
+      await shaka.test.Util.shortDelay();
+
+      expect(numberOfRatechangeEvents).toBe(5);
+    });
+
     it('in sequence mode', async () => {
       if (!deviceDetected.supportsSequenceMode()) {
         pending('Sequence mode is not supported by the platform.');