Fix scrollbar damage accumulation issue

Source/platform/scroll/ScrollableArea.h

 /*
  * Copyright (C) 2008, 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 186 matching lines @@
 
     virtual bool userInputScrollable(ScrollbarOrientation) const = 0;
     virtual bool shouldPlaceVerticalScrollbarOnLeft() const = 0;
 
     // Convenience functions
     int scrollPosition(ScrollbarOrientation orientation) { return orientation == HorizontalScrollbar ? scrollPosition().x() : scrollPosition().y(); }
     int minimumScrollPosition(ScrollbarOrientation orientation) { return orientation == HorizontalScrollbar ? minimumScrollPosition().x() : minimumScrollPosition().y(); }
     int maximumScrollPosition(ScrollbarOrientation orientation) { return orientation == HorizontalScrollbar ? maximumScrollPosition().x() : maximumScrollPosition().y(); }
     int clampScrollPosition(ScrollbarOrientation orientation, int pos)  { return std::max(std::min(pos, maximumScrollPosition(orientation)), minimumScrollPosition(orientation)); }
 
-    bool hasVerticalBarDamage() const { return m_hasVerticalBarDamage; }
-    bool hasHorizontalBarDamage() const { return m_hasHorizontalBarDamage; }
+    bool hasVerticalBarDamage() const { return !m_verticalBarDamage.isEmpty(); }
+    bool hasHorizontalBarDamage() const { return !m_horizontalBarDamage.isEmpty(); }
+    const IntRect& verticalBarDamage() const { return m_verticalBarDamage; }
+    const IntRect& horizontalBarDamage() const { return m_horizontalBarDamage; }
 
-    const IntRect& verticalBarDamage() const
+    void addScrollbarDamage(Scrollbar* scrollbar, const IntRect& rect)
     {
-        ASSERT(m_hasVerticalBarDamage);
-        return m_verticalBarDamage;
-    }
-
-    const IntRect& horizontalBarDamage() const
-    {
-        ASSERT(m_hasHorizontalBarDamage);
-        return m_horizontalBarDamage;
+        if (scrollbar == horizontalScrollbar())
+            m_horizontalBarDamage.unite(rect);
+        else
+            m_verticalBarDamage.unite(rect);
     }
 
     void resetScrollbarDamage()
     {
-        m_hasVerticalBarDamage = false;
-        m_hasHorizontalBarDamage = false;
+        m_verticalBarDamage = IntRect();
+        m_horizontalBarDamage = IntRect();
     }
+
     virtual GraphicsLayer* layerForContainer() const;
     virtual GraphicsLayer* layerForScrolling() const { return 0; }
     virtual GraphicsLayer* layerForHorizontalScrollbar() const { return 0; }
     virtual GraphicsLayer* layerForVerticalScrollbar() const { return 0; }
     virtual GraphicsLayer* layerForScrollCorner() const { return 0; }
     bool hasLayerForHorizontalScrollbar() const;
     bool hasLayerForVerticalScrollbar() const;
     bool hasLayerForScrollCorner() const;
 
     void cancelProgrammaticScrollAnimation();
 
 protected:
     ScrollableArea();
     virtual ~ScrollableArea();
 
     void setScrollOrigin(const IntPoint&);
     void resetScrollOriginChanged() { m_scrollOriginChanged = false; }
 
     virtual void invalidateScrollbarRect(Scrollbar*, const IntRect&) = 0;
     virtual void invalidateScrollCornerRect(const IntRect&) = 0;
 
-    // For repaint after layout, stores the damage to be repainted for the
-    // scrollbars.
-    unsigned m_hasHorizontalBarDamage : 1;
-    unsigned m_hasVerticalBarDamage : 1;
-    IntRect m_horizontalBarDamage;
-    IntRect m_verticalBarDamage;
-
 private:
     void scrollPositionChanged(const IntPoint&);
 
     // NOTE: Only called from the ScrollAnimator.
     friend class ScrollAnimator;
     void setScrollOffsetFromAnimation(const IntPoint&);
 
     // This function should be overriden by subclasses to perform the actual
     // scroll of the content.
     virtual void setScrollOffset(const IntPoint&) = 0;
 
     virtual int lineStep(ScrollbarOrientation) const;
     virtual int pageStep(ScrollbarOrientation) const;
     virtual int documentStep(ScrollbarOrientation) const;
     virtual float pixelStep(ScrollbarOrientation) const;
 
+    // For repaint after layout, stores the damage to be repainted for the

[dsinclair, 2014/07/25 14:33:45]

Since repaint-after-layout is the only invalidation scheme, let's re-write this.
To just be:

Stores the paint invalidations for the scrollbars.

[Xianzhu, 2014/07/25 17:28:25]

Done.

+    // scrollbars.
+    IntRect m_horizontalBarDamage;
+    IntRect m_verticalBarDamage;
+
     struct ScrollableAreaAnimators {
         OwnPtr<ScrollAnimator> scrollAnimator;
         OwnPtr<ProgrammaticScrollAnimator> programmaticScrollAnimator;
     };
 
     mutable OwnPtr<ScrollableAreaAnimators> m_animators;
     unsigned m_constrainsScrollingToContentEdge : 1;
 
     unsigned m_inLiveResize : 1;
 
@@ skipping 14 matching lines @@
     // vertical-lr / ltr            NO                      NO
     // vertical-lr / rtl            NO                      YES
     // vertical-rl / ltr            YES                     NO
     // vertical-rl / rtl            YES                     YES
     IntPoint m_scrollOrigin;
 };
 
 } // namespace blink
 
 #endif // ScrollableArea_h