Bug 779572: Factor zoom into min pan distance before moving the viewport r=cjones

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -238,10 +238,13 @@ nsEventStatus AsyncPanZoomController::OnTouchMove(const MultiTouchInput& aEvent)
     case TOUCHING: {
       float panThreshold = 1.0f/2.0f * mDPI;
       UpdateWithTouchAtDevicePoint(aEvent);
+
       if (PanDistance() < panThreshold) {
         return nsEventStatus_eIgnore;
       }
+
       StartPanning(aEvent);
+
       return nsEventStatus_eConsumeNoDefault;
     }
 
@@ -442,7 +445,8 @@ nsEventStatus AsyncPanZoomController::OnCancelTap(const TapGestureInput& aEvent)
 }
 
 float AsyncPanZoomController::PanDistance() {
-  return NS_hypot(mX.PanDistance(), mY.PanDistance()) * mFrameMetrics.mResolution.width;
+  MonitorAutoLock monitor(mMonitor);
+  return NS_hypot(mX.PanDistance(), mY.PanDistance());
 }
 
 const nsPoint AsyncPanZoomController::GetVelocityVector() {