Bug 594253. Update mValidRegion by OR-ing with the visible region after repainting, instead of OR-ing with the redrawn region, since this leads to simpler region structures. r=cjones,a=blocking

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;