Fix text track change edge cases

src/controller/subtitle-stream-controller.js

@@ -146,7 +146,15 @@ class SubtitleStreamController extends TaskLoop {
 
   onSubtitleTrackSwitch (data) {
     this.currentTrackId = data.id;
-    this.clearVttFragQueues();
+    if (this.currentTrackId === -1)
+      return;
+
+    // Check if track was already loaded and if so make sure we finish
+    // downloading its frags, if not all have been downloaded yet
+    const currentTrack = this.tracks[this.currentTrackId];
+    let details = currentTrack.details;
+    if (details !== undefined)
+      this.tick();
   }
 
   // Got a new set of subtitle fragments.

src/controller/timeline-controller.js

@@ -255,6 +255,13 @@ class TimelineController extends EventHandler {
     // Parse the WebVTT file contents.
     WebVTTParser.parse(payload, this.initPTS, vttCCs, frag.cc, function (cues) {
       const currentTrack = textTracks[frag.trackId];
+      // WebVTTParser.parse is an async method and if the currently selected text track mode is set to "disabled"
+      // before parsing is done then don't try to access currentTrack.cues.getCueById as cues will be null
+      // and trying to access getCueById method of cues will throw an exception
+      if (currentTrack.mode === 'disabled') {
+        hls.trigger(Event.SUBTITLE_FRAG_PROCESSED, { success: false, frag: frag });
+        return;
+      }
       // Add cues and trigger event with success true.
       cues.forEach(cue => {
         // Sometimes there are cue overlaps on segmented vtts so the same
@@ -269,7 +276,8 @@ class TimelineController extends EventHandler {
             currentTrack.addCue(textTrackCue);
           }
         }
-      });
+      }
+      );
       hls.trigger(Event.SUBTITLE_FRAG_PROCESSED, { success: true, frag: frag });
     },
     function (e) {

tests/functional/auto/setup.js

@@ -160,7 +160,7 @@ describe('testing hls.js playback in the browser on "' + browserDescription + '"
           window.switchToHighestLevel('next');
         };
         window.setTimeout(function () {
-          var readyState = video.readyState;
+          let readyState = video.readyState;
           console.log('[log] > readyState:' + readyState);
           callback({ code: readyState, logs: window.logString });
         }, 12000);