Bug 1016035 - Make APZEventState report defaultPrevented=true when the event started a swipe. r=kats

--HG--
extra : commitid : 4scVCanEMYd
extra : rebase_source : a9ff41cb1a7f681fae4b352b8b5337bf0ce41ff2

gfx/layers/apz/util/APZEventState.cpp

@@ -324,7 +324,11 @@ APZEventState::ProcessWheelEvent(const WidgetWheelEvent& aEvent,
                                  const ScrollableLayerGuid& aGuid,
                                  uint64_t aInputBlockId)
 {
-  mContentReceivedInputBlockCallback->Run(aGuid, aInputBlockId, aEvent.mFlags.mDefaultPrevented);
+  // If this event starts a swipe, indicate that it shouldn't result in a
+  // scroll by setting defaultPrevented to true.
+  bool defaultPrevented =
+    aEvent.mFlags.mDefaultPrevented || aEvent.TriggersSwipe();
+  mContentReceivedInputBlockCallback->Run(aGuid, aInputBlockId, defaultPrevented);
 }
 
 void