fix(cues): only copy cue props that don't exist (#4145)

src/js/tracks/text-track.js

@@ -343,7 +343,9 @@ class TextTrack extends Track {
       cue = new window.vttjs.VTTCue(originalCue.startTime, originalCue.endTime, originalCue.text);
 
       for (const prop in originalCue) {
-        cue[prop] = originalCue[prop];
+        if (!(prop in cue)) {
+          cue[prop] = originalCue[prop];
+        }
       }
     }
 

test/unit/tracks/text-track.test.js

@@ -19,7 +19,17 @@ const defaultTech = {
   currentTime() {}
 };
 
-QUnit.module('Text Track');
+QUnit.module('Text Track', {
+  beforeEach() {
+    this.oldVttjs = window.vttjs;
+    window.vttjs = {
+      VTTCue: Object
+    };
+  },
+  afterEach() {
+    window.vttjs = this.oldVttjs;
+  }
+});
 
 // do baseline track testing
 TrackBaseline(TextTrack, {