Bug 1148868 - Disable the workaround from bug 1151890 unless we use the separate event thread runner. r=kats

--HG--
extra : rebase_source : e8a643f88775023d6c29d0708f3e8baae58b8b95

widget/cocoa/nsChildView.mm

@@ -1896,9 +1896,11 @@ nsChildView::ConfigureAPZCTreeManager()
 void
 nsChildView::ConfigureAPZControllerThread()
 {
-  // On OS X the EventThreadRunner is the controller thread, but it doesn't
+  if (gfxPrefs::AsyncPanZoomSeparateEventThread()) {
+    // The EventThreadRunner is the controller thread, but it doesn't
     // have a MessageLoop.
     APZThreadUtils::SetControllerThread(nullptr);
+  }
 }
 
 nsIntRect