diff --git a/gfx/layers/basic/BasicLayers.cpp b/gfx/layers/basic/BasicLayers.cpp
index acc82ca82359..19730dd0a57b 100644
--- a/gfx/layers/basic/BasicLayers.cpp
+++ b/gfx/layers/basic/BasicLayers.cpp
@@ -423,7 +423,11 @@ protected:
     }
     aCallback(this, aContext, aRegionToDraw, aRegionToInvalidate,
               aCallbackData);
-    mValidRegion.Or(mValidRegion, aRegionToDraw);
+    // Everything that's visible has been validated. Do this instead of
+    // OR-ing with aRegionToDraw, since that can lead to a very complex region
+    // here (OR doesn't automatically simplify to the simplest possible
+    // representation of a region.)
+    mValidRegion.Or(mValidRegion, mVisibleRegion);
   }
 
   Buffer mBuffer;
diff --git a/gfx/layers/opengl/ThebesLayerOGL.cpp b/gfx/layers/opengl/ThebesLayerOGL.cpp
index de812d79114d..c4757bff8989 100644
--- a/gfx/layers/opengl/ThebesLayerOGL.cpp
+++ b/gfx/layers/opengl/ThebesLayerOGL.cpp
@@ -555,7 +555,11 @@ ThebesLayerOGL::RenderLayer(int aPreviousFrameBuffer,
     void* callbackData = mOGLManager->GetThebesLayerCallbackData();
     callback(this, state.mContext, state.mRegionToDraw,
              state.mRegionToInvalidate, callbackData);
-    mValidRegion.Or(mValidRegion, state.mRegionToDraw);
+    // Everything that's visible has been validated. Do this instead of
+    // OR-ing with aRegionToDraw, since that can lead to a very complex region
+    // here (OR doesn't automatically simplify to the simplest possible
+    // representation of a region.)
+    mValidRegion.Or(mValidRegion, mVisibleRegion);
   }
 
   DEBUG_GL_ERROR_CHECK(gl());