Backed out changeset ed7a32143aeb (bug 1041530) for OS X debug mochitest failures

layout/base/FrameLayerBuilder.cpp

@@ -2635,11 +2635,10 @@ ContainerState::ComputeOpaqueRect(nsDisplayItem* aItem,
         opaqueClipped.Contains(mContainerBounds)) {
       aList->SetIsOpaque();
     }
-    // Add opaque areas to the "exclude glass" region. Only do this when our
-    // container layer is going to be the rootmost layer, otherwise transforms
-    // etc will mess us up (and opaque contributions from other containers are
-    // not needed).
-    if (!nsLayoutUtils::GetCrossDocParentFrame(mContainerFrame)) {
+    // Add opaque areas to the "exclude glass" region. Only do this for
+    // ThebesLayers which are direct children of the root layer; this means
+    // they can't have transforms or opacity wrapping them.
+    if (!mContainerLayer->GetParent()) {
       mBuilder->AddWindowOpaqueRegion(opaqueClipped);
     }
     opaquePixels = ScaleRegionToInsidePixels(opaqueClipped, snapOpaque);