Fix several corner case issues of scrollbar paint invalidation

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 3390 matching lines @@
 
     // The PaintController may have changed. Pass the previous paint invalidation rect to the new PaintController.
     // The rect will be updated if it changes during the next paint invalidation.
     invalidateDisplayItemClients(paintInvalidationContainer, PaintInvalidationLayer, &invalidationRect);
 
     // This method may be used to invalidate paint of an object changing paint invalidation container.
     // Clear previous paint invalidation rect on the original paint invalidation container to avoid
     // under-invalidation if the new paint invalidation rect on the new paint invalidation container
     // happens to be the same as the old one.
     setPreviousPaintInvalidationRect(LayoutRect());
+    if (isBox()) {
+        if (PaintLayerScrollableArea* scrollableArea = toLayoutBox(this)->scrollableArea())
+            scrollableArea->clearPreviousPaintInvalidationRects();
+    }
 }
 
 void LayoutObject::invalidatePaintIncludingNonCompositingDescendants()
 {
     // Since we're only painting non-composited layers, we know that they all share the same paintInvalidationContainer.
     const LayoutBoxModelObject& paintInvalidationContainer = containerForPaintInvalidation();
     traverseNonCompositingDescendants(*this, [&paintInvalidationContainer](LayoutObject& object) {
         object.invalidatePaintOfPreviousPaintInvalidationRect(paintInvalidationContainer, PaintInvalidationLayer);
     });
 }
@@ skipping 78 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         fprintf(stderr, "Cannot showLayoutTree. Root is (nil)\n");
     }
 }
 
 #endif