fix: clear stuck subtitle track cues

src/plugins/htmlVideoPlayer/plugin.js

@@ -620,6 +620,28 @@ function tryRemoveElement(elem) {
             return relativeOffset;
         }
 
+        /**
+         * @private
+         * These browsers will not clear the existing active cue when setting an offset
+         * for native TextTracks.
+         * Any previous text tracks that are on the screen when the offset changes will
+         * remain next to the new tracks until they reach the new offset's instance of the track.
+         */
+        requiresHidingActiveCuesOnOffsetChange() {
+            return !!browser.firefox;
+        }
+
+        /**
+         * @private
+         */
+        hideTextTrackActiveCues(currentTrack) {
+            if (currentTrack.activeCues) {
+                Array.from(currentTrack.activeCues).forEach((cue) => {
+                    cue.text = '';
+                });
+            }
+        }
+
         /**
          * @private
          */
@@ -629,6 +651,9 @@ function tryRemoveElement(elem) {
                 if (offsetValue === 0) {
                     return;
                 }
+                if (this.requiresHidingActiveCuesOnOffsetChange()) {
+                    this.hideTextTrackActiveCues(currentTrack);
+                }
                 Array.from(currentTrack.cues)
                     .forEach(function (cue) {
                         cue.startTime -= offsetValue;