Don't pass subpixel offsets through non-translation transforms (composited case)

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 874 matching lines @@
     LayoutPoint& offset_from_composited_ancestor,
     IntPoint& snapped_offset_from_composited_ancestor) {
   LayoutRect local_raw_compositing_bounds = CompositedBounds();
   offset_from_composited_ancestor = ComputeOffsetFromCompositedAncestor(
       &owning_layer_, composited_ancestor,
       local_raw_compositing_bounds.Location());
   snapped_offset_from_composited_ancestor =
       IntPoint(offset_from_composited_ancestor.X().Round(),
                offset_from_composited_ancestor.Y().Round());
 
-  LayoutSize subpixel_accumulation =
-      offset_from_composited_ancestor - snapped_offset_from_composited_ancestor;
+  LayoutSize subpixel_accumulation;
+  if (!owning_layer_.Transform() ||
+      owning_layer_.Transform()->IsIdentityOrTranslation()) {
+    subpixel_accumulation = offset_from_composited_ancestor -
+                            snapped_offset_from_composited_ancestor;
+  }
+  // Otherwise discard the sub-pixel remainder because paint offset can't be
+  // transformed by a non-translation transform.
   owning_layer_.SetSubpixelAccumulation(subpixel_accumulation);
 
   // Move the bounds by the subpixel accumulation so that it pixel-snaps
   // relative to absolute pixels instead of local coordinates.
   local_raw_compositing_bounds.Move(subpixel_accumulation);
   local_bounds = PixelSnappedIntRect(local_raw_compositing_bounds);
 
   compositing_bounds_relative_to_composited_ancestor = local_bounds;
   compositing_bounds_relative_to_composited_ancestor.MoveBy(
       snapped_offset_from_composited_ancestor);
@@ skipping 2708 matching lines @@
   } else if (graphics_layer == decoration_outline_layer_.get()) {
     name = "Decoration Layer";
   } else {
     NOTREACHED();
   }
 
   return name;
 }
 
 }  // namespace blink