--- a/gfx/layers/composite/LayerManagerComposite.cpp
+++ b/gfx/layers/composite/LayerManagerComposite.cpp
@@ -221,17 +221,21 @@ LayerManagerComposite::ApplyOcclusionCul
     }
   }
 
   // Subtract any areas that we know to be opaque from our
   // visible region.
   LayerComposite *composite = aLayer->AsLayerComposite();
   if (!localOpaque.IsEmpty()) {
     nsIntRegion visible = composite->GetShadowVisibleRegion();
-    visible.Sub(visible, localOpaque);
+    nsIntRegion afterCulling;
+    afterCulling.Sub(visible, localOpaque);
+    // Intersect the original region with the bounds of the culled region so
+    // that we don't increase the region's complexity.
+    visible.AndWith(afterCulling.GetBounds());
     composite->SetShadowVisibleRegion(visible);
   }
 
   // Compute occlusions for our descendants (in front-to-back order) and allow them to
   // contribute to localOpaque.
   for (Layer* child = aLayer->GetLastChild(); child; child = child->GetPrevSibling()) {
     ApplyOcclusionCulling(child, localOpaque);
   }