Simplify computation of the invalidation rect for a frame.

third_party/WebKit/Source/core/paint/PaintLayerClipper.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 129 matching lines @@
 // #container and thus 'overflow' doesn't apply to it. During our tree walk,
 // #fixed is a child of #container, which is the reason why we keep 3 clip rects
 // depending on the 'position' of the elements.
 //
 // Now instead if we add "clip: rect(0px, 100px, 100px, 0px)" to #container,
 // the clip will apply to both #inflow and #fixed. That's because 'clip'
 // applies to any descendant, regardless of containing blocks. Note that
 // #container and #fixed are siblings in the paint tree but #container does
 // clip #fixed. This is the reason why we compute the painting clip rects during
 // a layout tree walk and cache them for painting.
-class PaintLayerClipper {
+class CORE_EXPORT PaintLayerClipper {
     DISALLOW_NEW();
     WTF_MAKE_NONCOPYABLE(PaintLayerClipper);
 public:
     explicit PaintLayerClipper(const LayoutBoxModelObject&);
 
     void clearClipRectsIncludingDescendants();
     void clearClipRectsIncludingDescendants(ClipRectsCacheSlot);
 
-    LayoutRect childrenClipRect() const; // Returns the foreground clip rect of the layer in the document's coordinate space.
+    LayoutRect childrenClipRect() const; // Returns the foreground clip rect of the layer in the absolute coordinate space.
     LayoutRect localClipRect() const; // Returns the background clip rect of the layer in the local coordinate space.
 
     ClipRect backgroundClipRect(const ClipRectsContext&) const;
 
     // This method figures out our layerBounds in coordinates relative to
     // |rootLayer|. It also computes our background and foreground clip rects
     // for painting/event handling.
     // Pass offsetFromRoot if known.
     void calculateRects(const ClipRectsContext&, const LayoutRect& paintDirtyRect, LayoutRect& layerBounds,
         ClipRect& backgroundRect, ClipRect& foregroundRect, const LayoutPoint* offsetFromRoot = 0) const;
@@ skipping 26 matching lines @@
     // FIXME: Could this be a LayoutBox?
     const LayoutBoxModelObject& m_layoutObject;
 
     // Lazily created by 'cache() const'.
     mutable OwnPtr<ClipRectsCache> m_cache;
 };
 
 } // namespace blink
 
 #endif // LayerClipper_h