Fix several corner case issues of scrollbar paint invalidation

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 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 318 matching lines @@
     CompositingInputsUpdater::assertNeedsCompositingInputsUpdateBitsCleared(rootLayer());
 #endif
 }
 
 static void forceRecomputePaintInvalidationRectsIncludingNonCompositingDescendants(LayoutObject* layoutObject)
 {
     // We clear the previous paint invalidation rect as it's wrong (paint invaliation container
     // changed, ...). Forcing a full invalidation will make us recompute it. Also we are not
     // changing the previous position from our paint invalidation container, which is fine as
     // we want a full paint invalidation anyway.
     layoutObject->setPreviousPaintInvalidationRect(LayoutRect());

[chrishtr, 2015/12/03 18:52:36]

Add a new virtual method LayoutObject::clearPreviousPaintInvalidationRects() and
override in LayoutBox?

This would avoid the isBox() stuff, and make the intent of the code here and the
other place clearer.

[Xianzhu, 2015/12/03 22:13:27]

Done.

+    if (layoutObject->isBox()) {
+        if (PaintLayerScrollableArea* scrollableArea = toLayoutBox(layoutObject)->scrollableArea())
+            scrollableArea->clearPreviousPaintInvalidationRects();
+    }
     layoutObject->setShouldDoFullPaintInvalidation();
 
     for (LayoutObject* child = layoutObject->slowFirstChild(); child; child = child->nextSibling()) {
         if (!child->isPaintInvalidationContainer())
             forceRecomputePaintInvalidationRectsIncludingNonCompositingDescendants(child);
     }
 }
 
 
 void PaintLayerCompositor::updateIfNeeded()
@@ skipping 842 matching lines @@
     } else if (graphicsLayer == m_scrollLayer.get()) {
         name = "Frame Scrolling Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink