Bug 1495588 - Prevent the parent frame from getting handled UA Widget events r=kmag,smaug

This can happen when the message manager is associated to an embedded frame,
like an <iframe mozbrowser>.

Differential Revision: https://phabricator.services.mozilla.com/D7362

--HG--
extra : moz-landing-system : lando

browser/actors/UAWidgetsChild.jsm

@@ -26,6 +26,10 @@ class UAWidgetsChild extends ActorChild {
         this.teardownWidget(aEvent.target);
         break;
     }
+
+    // In case we are a nested frame, prevent the message manager of the
+    // parent frame from receving the event.
+    aEvent.stopPropagation();
   }
 
   setupOrNotifyWidget(aElement) {