Aura overlay scrollbars shouldn't invalidate thumb on scroll or enabled state.

third_party/WebKit/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 513 matching lines @@
 void Scrollbar::setScrollbarsHidden(bool hidden) {
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarsHidden(hidden);
 }
 
 void Scrollbar::setEnabled(bool e) {
   if (m_enabled == e)
     return;
   m_enabled = e;
   theme().updateEnabledState(*this);
-  setNeedsPaintInvalidation(AllParts);
+
+  ScrollbarPart invalidParts = theme().invalidateOnEnabledChange();
+  setNeedsPaintInvalidation(invalidParts);
 }
 
 int Scrollbar::scrollbarThickness() const {
   int thickness = orientation() == HorizontalScrollbar ? height() : width();
   if (!thickness || !m_hostWindow)
     return thickness;
   return m_hostWindow->windowToViewportScalar(m_themeScrollbarThickness);
 }
 
 bool Scrollbar::isOverlayScrollbar() const {
@@ skipping 82 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink