--- a/gfx/layers/client/ClientTiledPaintedLayer.h
+++ b/gfx/layers/client/ClientTiledPaintedLayer.h
@@ -50,18 +50,19 @@ protected:
 public:
   // Override name to distinguish it from ClientPaintedLayer in layer dumps
   virtual const char* Name() const { return "TiledPaintedLayer"; }
 
   // PaintedLayer
   virtual Layer* AsLayer() { return this; }
   virtual void InvalidateRegion(const nsIntRegion& aRegion) {
     mInvalidRegion.Or(mInvalidRegion, aRegion);
-    mValidRegion.Sub(mValidRegion, aRegion);
-    mLowPrecisionValidRegion.Sub(mLowPrecisionValidRegion, aRegion);
+    mInvalidRegion.SimplifyOutward(20);
+    mValidRegion.Sub(mValidRegion, mInvalidRegion);
+    mLowPrecisionValidRegion.Sub(mLowPrecisionValidRegion, mInvalidRegion);
   }
 
   // Shadow methods
   virtual void FillSpecificAttributes(SpecificLayerAttributes& aAttrs);
   virtual ShadowableLayer* AsShadowableLayer() { return this; }
 
   virtual void Disconnect()
   {