Bug 1031067 - Clear overscroll at the end of a pinch. r=kats

--HG--
extra : rebase_source : 2484d2513aab9d9d321fe6d42a1face685b45ba0

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -808,6 +808,10 @@ APZCTreeManager::DispatchScroll(AsyncPanZoomController* aPrev, ScreenPoint aStar
     return false;
   }
 
+  if (next->GetGuid().mLayersId != aPrev->GetGuid().mLayersId) {
+    NS_WARNING("Handing off scroll across a layer tree boundary; may need to revise approach to bug 1031067");
+  }
+
   // Convert the start and end points from |aPrev|'s coordinate space to
   // |next|'s coordinate space. Since |aPrev| may be the same as |next|
   // (if |aPrev| is the APZC that is initiating the scroll and there is no

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -1240,6 +1240,15 @@ nsEventStatus AsyncPanZoomController::OnScaleEnd(const PinchGestureInput& aEvent
 
   {
     ReentrantMonitorAutoEnter lock(mMonitor);
+
+    // We can get into a situation where we are overscrolled at the end of a
+    // pinch if we go into overscroll with a two-finger pan, and then turn
+    // that into a pinch by increasing the span sufficiently. In such a case,
+    // there is no snap-back animation to get us out of overscroll, so we need
+    // to get out of it somehow.
+    mX.ClearOverscroll();
+    mY.ClearOverscroll();
+
     ScheduleComposite();
     RequestContentRepaint();
     UpdateSharedCompositorFrameMetrics();