--- a/gfx/layers/composite/LayerManagerComposite.cpp
+++ b/gfx/layers/composite/LayerManagerComposite.cpp
@@ -273,16 +273,19 @@ LayerManagerComposite::PostProcessLayers
                "Only direct children of the establisher could have a clip");
 
     for (Layer* child = aLayer->GetLastChild();
          child;
          child = child->GetPrevSibling()) {
       PostProcessLayers(child, aOpaqueRegion, aVisibleRegion,
                         ancestorClipForChildren);
     }
+    if (layerClip) {
+      aOpaqueRegion.AndWith(layerClip->ToUnknownRect());
+    }
     return;
   }
 
   nsIntRegion localOpaque;
   // Treat layers on the path to the root of the 3D rendering context as
   // a giant layer if it is a leaf.
   Matrix4x4 transform = GetAccTransformIn3DContext(aLayer);
   Matrix transform2d;