Bug 510856. Round out 'visible area of moving content' to device pixel edges to avoid stray subpixel rectangles needing to be repainted. r=dbaron

--HG--
extra : rebase_source : 83e0e490c4b145fb774ad149c7f7a9e6ff8998c7

layout/base/nsDisplayList.cpp

@@ -364,8 +364,11 @@ nsDisplayList::OptimizeVisibility(nsDisplayListBuilder* aBuilder,
         // the first one we see is a sound overapproximation
         movingContentVisibleRegion = *aVisibleRegion;
       }
+      nscoord appUnitsPerPixel = f->PresContext()->AppUnitsPerDevPixel();
+      nsRect bounds = item->GetBounds(aBuilder).
+          ToOutsidePixels(appUnitsPerPixel).ToAppUnits(appUnitsPerPixel);
       movingContentAccumulatedBounds.UnionRect(movingContentAccumulatedBounds,
-                                               item->GetBounds(aBuilder));
+                                               bounds);
     }
     if (item->OptimizeVisibility(aBuilder, aVisibleRegion)) {
       AppendToBottom(item);