Conditionally create PaintLayer's scrollable area object

third_party/WebKit/Source/core/layout/LayoutScrollbarPart.cpp

 /*
  * Copyright (C) 2008 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 162 matching lines @@
 }
 
 void LayoutScrollbarPart::styleDidChange(StyleDifference diff, const ComputedStyle* oldStyle)
 {
     LayoutBlock::styleDidChange(diff, oldStyle);
     // See adjustStyleBeforeSet() above.
     ASSERT(!isOrthogonalWritingModeRoot());
     setInline(false);
     clearPositionedState();
     setFloating(false);
-    setHasOverflowClip(false);

[Xianzhu, 2016/02/25 19:57:51]

This is changed to use allowsOverflowClip() method so that we can have
consistent hasOverflowClip() during setStyle(). Previously we may have temporary
true hasOverflowClip after LayoutBox::updateFromStyle() before this
styleDidChange(), causing unnecessary PaintLayer creation for objects not
applicable of overflow clip.

     if (oldStyle && (diff.needsPaintInvalidation() || diff.needsLayout()))
         setNeedsPaintInvalidation();
 }
 
 void LayoutScrollbarPart::imageChanged(WrappedImagePtr image, const IntRect* rect)
 {
     setNeedsPaintInvalidation();
     LayoutBlock::imageChanged(image, rect);
 }
 
@@ skipping 16 matching lines @@
             frameView->setScrollCornerNeedsPaintInvalidation();
             return;
         }
     }
 
     // This LayoutScrollbarPart belongs to a PaintLayerScrollableArea.
     toLayoutBox(parent())->scrollableArea()->setScrollCornerNeedsPaintInvalidation();
 }
 
 } // namespace blink