Bug 970070 - Update the last-dispatched-paint metrics when we get a scroll update from layout. r=botond

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -1705,6 +1705,12 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
       // scroll offset updates from APZ until we acknowledge the update it sent.
       // This prevents APZ updates from clobbering scroll updates from other
       // more "legitimate" sources like content scripts.
+      // Furthermore, any inflight paint requests we have already dispatched are
+      // going to be ignored by layout, and so mLastDispatchedPaintMetrics
+      // becomes incorrect for the purposes of calculating the LD transform. To
+      // correct this we need to update mLastDispatchedPaintMetrics to be the
+      // last thing we know was painted by Gecko.
+      mLastDispatchedPaintMetrics = aLayerMetrics;
       nsRefPtr<GeckoContentController> controller = GetGeckoContentController();
       if (controller) {
         controller->AcknowledgeScrollUpdate(aLayerMetrics.mScrollId,