Fix invalidation during painting of scrollbars

Source/platform/scroll/Scrollbar.cpp

 /*
  * Copyright (C) 2004, 2006, 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 87 matching lines @@
         return;
 
     ASSERT(m_scrollableArea);
     if (m_orientation == VerticalScrollbar)
         m_animator->willRemoveVerticalScrollbar(this);
     else
         m_animator->willRemoveHorizontalScrollbar(this);
 #endif
 }
 
+void Scrollbar::setFrameRect(const IntRect& frameRect)
+{
+    if (frameRect != this->frameRect()) {

[Julien - ping for review, 2015/02/25 01:43:36]

We prefer early return whenever possible.

[Xianzhu, 2015/02/25 01:57:09]

Done.

+        invalidate();

[chrishtr, 2015/02/24 22:38:42]

Is this because widgets don't obey the standard way of doing paint invalidation?

[Xianzhu, 2015/02/24 23:01:56]

Yes. They are not LayoutObjects, so the normal
previousPaintInvalidationRect/needsFullPaintInvalidation mechanisms don't apply
for them.

+        Widget::setFrameRect(frameRect);
+        invalidate();
+    }
+}
+
 ScrollbarOverlayStyle Scrollbar::scrollbarOverlayStyle() const
 {
     return m_scrollableArea ? m_scrollableArea->scrollbarOverlayStyle() : ScrollbarOverlayStyleDefault;
 }
 
 void Scrollbar::getTickmarks(Vector<IntRect>& tickmarks) const
 {
     if (m_scrollableArea)
         m_scrollableArea->getTickmarks(tickmarks);
 }
@@ skipping 437 matching lines @@
 
     return m_scrollableArea->scrollPosition().y() - m_scrollableArea->minimumScrollPosition().y();
 }
 
 DisplayItemClient Scrollbar::displayItemClient() const
 {
     return m_scrollableArea->displayItemClient();
 }
 
 } // namespace blink