patch to Bug 1363423

layout/base/FrameLayerBuilder.cpp

@@ -3649,6 +3649,10 @@ PaintedLayerData::AccumulateEventRegions(ContainerState* aState, nsDisplayLayerE
     mDispatchToContentHitRegion.OrWith(CombinedTouchActionRegion());
   }
   
+  // Avoid quadratic performance as a result of the region growing to include
+  // and arbitrarily large number of rects, which can happen on some pages.
+  mMaybeHitRegion.SimplifyOutward(8);
+
   // Calculate scaled versions of the bounds of mHitRegion and mMaybeHitRegion
   // for quick access in FindPaintedLayerFor().
   mScaledHitRegionBounds = aState->ScaleToOutsidePixels(mHitRegion.GetBounds());
@@ -6368,3 +6372,4 @@ ContainerState::CreateMaskLayer(Layer *aLayer,
 }
 
 } // namespace mozilla
+