Bug 1216355 - Ensure that if a repaint request is ignored due to a stale generation number the next repaint request doesn't get dropped. r=botond

--HG--
extra : commitid : 5Zh7BCHsJWF

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2741,8 +2741,11 @@ void AsyncPanZoomController::RequestContentRepaint(FrameMetrics& aFrameMetrics,
       fabsf(mLastPaintRequestMetrics.GetScrollOffset().y -
             aFrameMetrics.GetScrollOffset().y) < EPSILON &&
       aFrameMetrics.GetZoom() == mLastPaintRequestMetrics.GetZoom() &&
-      fabsf(aFrameMetrics.GetViewport().width - mLastPaintRequestMetrics.GetViewport().width) < EPSILON &&
-      fabsf(aFrameMetrics.GetViewport().height - mLastPaintRequestMetrics.GetViewport().height) < EPSILON) {
+      fabsf(aFrameMetrics.GetViewport().width -
+            mLastPaintRequestMetrics.GetViewport().width) < EPSILON &&
+      fabsf(aFrameMetrics.GetViewport().height -
+            mLastPaintRequestMetrics.GetViewport().height) < EPSILON &&
+      aFrameMetrics.GetScrollGeneration() == mLastPaintRequestMetrics.GetScrollGeneration()) {
     return;
   }
 
@@ -3190,8 +3193,14 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
       Stringify(mFrameMetrics.GetScrollOffset()).c_str(),
       Stringify(aLayerMetrics.GetSmoothScrollOffset()).c_str());
 
-    mFrameMetrics.CopySmoothScrollInfoFrom(aLayerMetrics);
     CancelAnimation();
+
+    // It's important to not send repaint requests between this
+    // CopySmoothScrollInfo call and the sending of the scroll update
+    // acknowledgement below, otherwise that repaint request will get rejected
+    // on the main thread but mLastPaintRequestMetrics will have the new scroll
+    // generation; this would leave the main thread out of date.
+    mFrameMetrics.CopySmoothScrollInfoFrom(aLayerMetrics);
     mLastDispatchedPaintMetrics = aLayerMetrics;
     StartSmoothScroll(ScrollSource::DOM);