fix: Text tracks with src= on Safari & UITextDisplayer are sometimes rendered natively (#8256)

This concerns `src=` mode. Safari loads text tracks separately, where on
`loadeddata`, we might not have `video.textTracks` set to its final
value (as Safari could still be processing another text track
underneath).

This PR introduces an artificial wait on the `addtrack` and assumes, if
no other track is added within the next 500ms, textTracks shall be in
its final state.

Issues when we work with `textTracks` too early:
- When we set mode of a textTrack to `hidden` (we'd do that to allow
UiTextDisplayer to render text tracks), Safari might overwrite this
decision (based on `AUTOSELECT=YES`) when a subsequent text track is
parsed. This leaves us in an invalid state where textTracks are rendered
natively but we have listeners bound to the wrong one. More info here:
#8255
- It could be no textTracks are available at all on `loadeddata` due to
Safari internally defering the load for text tracks. This would break
preferences.

---------

Co-authored-by: Álvaro Velad Galván <[email protected]>

Author: matvp91

lib/player.js

@@ -3092,41 +3092,41 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
           fullyLoaded.resolve();
         });
 
+    const waitForNativeTracks = () => {
+      return new Promise((resolve) => {
+        const GRACE_PERIOD = 0.5;
+        const timer = new shaka.util.Timer(resolve);
+        // Applying the text preference too soon can result in it being
+        // reverted.  Wait for native HLS to pick something first.
+        this.loadEventManager_.listen(mediaElement.textTracks,
+            'change', () => timer.tickAfter(GRACE_PERIOD));
+        timer.tickAfter(GRACE_PERIOD);
+      });
+    };
+
     // We can't switch to preferred languages, though, until the data is
     // loaded.
     shaka.util.MediaReadyState.waitForReadyState(mediaElement,
         HTMLMediaElement.HAVE_CURRENT_DATA,
         this.loadEventManager_,
         async () => {
+          await waitForNativeTracks();
+          // If we have moved on to another piece of content while waiting for
+          // the above event/timer, we should not change tracks here.
+          if (unloaded) {
+            return;
+          }
+
           this.setupPreferredAudioOnSrc_();
 
-          // Applying the text preference too soon can result in it being
-          // reverted.  Wait for native HLS to pick something first.
           const textTracks = this.getFilteredTextTracks_();
-          if (!textTracks.find((t) => t.mode != 'disabled')) {
-            await new Promise((resolve) => {
-              this.loadEventManager_.listenOnce(
-                  mediaElement.textTracks, 'change', resolve);
-
-              // We expect the event to fire because it does on Safari.
-              // But in case it doesn't on some other platform or future
-              // version, move on in 1 second no matter what.  This keeps the
-              // language settings from being completely ignored if something
-              // goes wrong.
-              new shaka.util.Timer(resolve).tickAfter(1);
-            });
-          } else if (textTracks.length > 0) {
+          // If Safari native picked one for us, we'll set text visible.
+          if (textTracks.some((t) => t.mode === 'showing')) {
             this.isTextVisible_ = true;
             this.textDisplayer_.setTextVisibility(true);
           }
 
-          // If we have moved on to another piece of content while waiting for
-          // the above event/timer, we should not change tracks here.
-          if (unloaded) {
-            return;
-          }
-
-          if (this.getFilteredTextTracks_().length) {
+          if (textTracks.length) {
             if (this.textDisplayer_.enableTextDisplayer) {
               this.textDisplayer_.enableTextDisplayer();
             } else {