Fix pixel snapping issues when layers become composited

Source/core/rendering/RenderLayerBacking.h

 /*
  * 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 105 matching lines @@
     void transitionPaused(double timeOffset, CSSPropertyID);
     void transitionFinished(CSSPropertyID);
 
     bool startAnimation(double timeOffset, const CSSAnimationData* anim, const KeyframeList& keyframes);
     void animationPaused(double timeOffset, const String& name);
     void animationFinished(const String& name);
 
     void suspendAnimations(double time = 0);
     void resumeAnimations();
 
-    IntRect compositedBounds() const;
-    void setCompositedBounds(const IntRect&);
+    LayoutRect compositedBounds() const;
+    void setCompositedBounds(const LayoutRect&);
     void updateCompositedBounds();
 
     void updateAfterWidgetResize();
     void positionOverflowControlsLayers(const IntSize& offsetFromRoot);
     bool hasUnpositionedOverflowControlsLayers() const;
 
     // GraphicsLayerClient interface
     virtual void notifyAnimationStarted(const GraphicsLayer*, double startTime) OVERRIDE;
 
     virtual void paintContents(const GraphicsLayer*, GraphicsContext&, GraphicsLayerPaintingPhase, const IntRect& clip) OVERRIDE;
 
     virtual void didCommitChangesForLayer(const GraphicsLayer*) const OVERRIDE;
     virtual bool getCurrentTransform(const GraphicsLayer*, TransformationMatrix&) const OVERRIDE;
 
     virtual bool isTrackingRepaints() const OVERRIDE;
 
 #ifndef NDEBUG
     virtual void verifyNotPainting();
 #endif
 
-    IntRect contentsBox() const;
+    LayoutRect contentsBox() const;
     IntRect backgroundBox() const;
 
     // For informative purposes only.
     CompositingLayerType compositingLayerType() const;
 
     GraphicsLayer* layerForHorizontalScrollbar() const { return m_layerForHorizontalScrollbar.get(); }
     GraphicsLayer* layerForVerticalScrollbar() const { return m_layerForVerticalScrollbar.get(); }
     GraphicsLayer* layerForScrollCorner() const { return m_layerForScrollCorner.get(); }
 
     void updateFilters(const RenderStyle*);
@@ skipping 23 matching lines @@
     bool requiresVerticalScrollbarLayer() const { return m_owningLayer->verticalScrollbar(); }
     bool requiresScrollCornerLayer() const { return !m_owningLayer->scrollCornerAndResizerRect().isEmpty(); }
     bool updateScrollingLayers(bool scrollingLayers);
     void updateDrawsContent(bool isSimpleContainer);
     void registerScrollingLayers();
 
     void setBackgroundLayerPaintsFixedRootBackground(bool);
 
     GraphicsLayerPaintingPhase paintingPhaseForPrimaryLayer() const;
 
-    IntSize contentOffsetInCompostingLayer() const;
+    LayoutSize contentOffsetInCompostingLayer() const;
     // Result is transform origin in pixels.
     FloatPoint3D computeTransformOrigin(const IntRect& borderBox) const;
     // Result is perspective origin in pixels.
     FloatPoint computePerspectiveOrigin(const IntRect& borderBox) const;
 
     void updateOpacity(const RenderStyle*);
     void updateTransform(const RenderStyle*);
     void updateLayerBlendMode(const RenderStyle*);
     // Return the opacity value that this layer should use for compositing.
     float compositingOpacity(float rendererOpacity) const;
@@ skipping 37 matching lines @@
 
     OwnPtr<GraphicsLayer> m_layerForHorizontalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 
     OwnPtr<GraphicsLayer> m_scrollingLayer; // Only used if the layer is using composited scrolling.
     OwnPtr<GraphicsLayer> m_scrollingContentsLayer; // Only used if the layer is using composited scrolling.
 
     uint64_t m_scrollLayerID;
 
-    IntRect m_compositedBounds;
+    LayoutRect m_compositedBounds;
+    LayoutSize m_subpixelAccumulation; // The accumulated subpixel offset of the compositedBounds compared to absolute coordinates.
 
     bool m_artificiallyInflatedBounds; // bounds had to be made non-zero to make transform-origin work
     bool m_boundsConstrainedByClipping;
     bool m_isMainFrameRenderViewLayer;
     bool m_requiresOwnBackingStore;
     bool m_canCompositeFilters;
     bool m_backgroundLayerPaintsFixedRootBackground;
 };
 
 } // namespace WebCore
 
 #endif // RenderLayerBacking_h