Issue #2030 - Dispatch click on common interactive ancestor if mousdown/up are not on the same element

For example, if the mouse is dragged from one element to another while
staying inside the same button, dispatch from their common ancestor

widget/MouseEvents.h

@@ -275,6 +275,10 @@ public:
     return result;
   }
 
+  // If during mouseup handling we detect that click event might need to be
+  // dispatched, this is setup to be the target of the click event.
+  nsCOMPtr<dom::EventTarget> mClickTarget;
+
   // mReason indicates the reason why the event is fired:
   // - Representing mouse operation.
   // - Synthesized for emulating mousemove event when the content under the