Bug 1180899 - Do not clobber frame metrics on the root layer added during a previous paint if there is no other frame metrics for the root scroll frame. r=tn

--HG--
extra : rebase_source : 25f54891b69373fd570ad86f9e2aaed852c10d03
extra : source : b0c9f2c474e9b446fb008023c6e7f423931e59cd

layout/base/nsDisplayList.cpp

@@ -1541,6 +1541,13 @@ already_AddRefed<LayerManager> nsDisplayList::PaintRoot(nsDisplayListBuilder* aB
     props = Move(LayerProperties::CloneFrom(layerManager->GetRoot()));
   }
 
+  // Clear any FrameMetrics that may have been set on the root layer on a
+  // previous paint. This paint will set new metrics if necessary, and if we
+  // don't clear the old one here, we may be left with extra metrics.
+  if (Layer* root = layerManager->GetRoot()) {
+      root->SetFrameMetrics(nsTArray<FrameMetrics>());
+  }
+
   ContainerLayerParameters containerParameters
     (presShell->GetResolution(), presShell->GetResolution());
   nsRefPtr<ContainerLayer> root = layerBuilder->
@@ -1616,9 +1623,6 @@ already_AddRefed<LayerManager> nsDisplayList::PaintRoot(nsDisplayListBuilder* aB
                          aBuilder->FindReferenceFrameFor(frame),
                          root, FrameMetrics::NULL_SCROLL_ID, viewport, Nothing(),
                          isRootContent, containerParameters));
-  } else {
-    // Set empty metrics to clear any metrics that might be on a recycled layer.
-    root->SetFrameMetrics(nsTArray<FrameMetrics>());
   }
 
   // NS_WARNING is debug-only, so don't even bother checking the conditions in