Bug 1130129 - Better handle a case where the APZ hit-test disagrees with the main-thread hit-test results. r=botond

dom/ipc/TabParent.cpp

@@ -2419,6 +2419,17 @@ TabParent::MaybeForwardEventToRenderFrame(WidgetInputEvent& aEvent,
     // id.
     if (aOutTargetGuid) {
       *aOutTargetGuid = InputAPZContext::GetTargetLayerGuid();
+
+      // There may be cases where the APZ hit-testing code came to a different
+      // conclusion than the main-thread hit-testing code as to where the event
+      // is destined. In such cases the layersId of the APZ result may not match
+      // the layersId of this renderframe. In such cases the main-thread hit-
+      // testing code "wins" so we need to update the guid to reflect this.
+      if (RenderFrameParent* rfp = GetRenderFrame()) {
+        if (aOutTargetGuid->mLayersId != rfp->GetLayersId()) {
+          *aOutTargetGuid = ScrollableLayerGuid(rfp->GetLayersId(), 0, FrameMetrics::NULL_SCROLL_ID);
+        }
+      }
     }
     if (aOutInputBlockId) {
       *aOutInputBlockId = InputAPZContext::GetInputBlockId();