change fullscreen element to player rootnode (#2204)

Co-authored-by: Walter van den Houten <63712489+wdfvdhouten@users.noreply.github.com>

src/js/fullscreen.js

@@ -124,7 +124,7 @@ class Fullscreen {
       return hasClass(this.target, this.player.config.classNames.fullscreen.fallback);
     }
 
-    const element = !this.prefix ? document.fullscreenElement : document[`${this.prefix}${this.property}Element`];
+    const element = !this.prefix ? this.target.getRootNode().fullscreenElement : this.target.getRootNode()[`${this.prefix}${this.property}Element`];
 
     return element && element.shadowRoot ? element === this.target.getRootNode().host : element === this.target;
   }