Fix pixel snapping of clipLayers and their descendants

Source/core/rendering/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 678 matching lines @@
     IntRect localCompositingBounds;
     IntRect relativeCompositingBounds;
     IntPoint delta;
     adjustBoundsForSubPixelAccumulation(compAncestor, localCompositingBounds, relativeCompositingBounds, delta);
 
     IntPoint graphicsLayerParentLocation;
     if (compAncestor && compAncestor->compositedLayerMapping()->hasClippingLayer()) {
         // If the compositing ancestor has a layer to clip children, we parent in that, and therefore
         // position relative to it.
         IntRect clippingBox = clipBox(toRenderBox(compAncestor->renderer()));
-        graphicsLayerParentLocation = clippingBox.location();
+        graphicsLayerParentLocation = clippingBox.location() + roundedIntSize(compAncestor->compositedLayerMapping()->subpixelAccumulation());
     } else if (compAncestor) {
         graphicsLayerParentLocation = ancestorCompositingBounds.location();
     } else {
         graphicsLayerParentLocation = renderer()->view()->documentRect().location();
     }
 
     if (compAncestor && compAncestor->needsCompositedScrolling()) {
         RenderBox* renderBox = toRenderBox(compAncestor->renderer());
         IntSize scrollOffset = renderBox->scrolledContentOffset();
         IntPoint scrollOrigin(renderBox->borderLeft(), renderBox->borderTop());
@@ skipping 20 matching lines @@
     m_graphicsLayer->setOffsetFromRenderer(toIntSize(localCompositingBounds.location()));
 
     FloatSize oldSize = m_graphicsLayer->size();
     if (oldSize != contentsSize)
         m_graphicsLayer->setSize(contentsSize);
 
     // If we have a layer that clips children, position it.
     IntRect clippingBox;
     if (GraphicsLayer* clipLayer = clippingLayer()) {
         clippingBox = clipBox(toRenderBox(renderer()));
-        clipLayer->setPosition(FloatPoint(clippingBox.location() - localCompositingBounds.location()));
+        clipLayer->setPosition(FloatPoint(clippingBox.location() - localCompositingBounds.location() + roundedIntSize(m_subpixelAccumulation)));
         clipLayer->setSize(clippingBox.size());
         clipLayer->setOffsetFromRenderer(toIntSize(clippingBox.location()));
         if (m_childClippingMaskLayer && !m_scrollingLayer) {
             m_childClippingMaskLayer->setPosition(clipLayer->position());
             m_childClippingMaskLayer->setSize(clipLayer->size());
             m_childClippingMaskLayer->setOffsetFromRenderer(clipLayer->offsetFromRenderer());
         }
     } else if (m_childTransformLayer) {
         m_childTransformLayer->setSize(contentsSize);
     }
@@ skipping 1395 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore