Reland: Make ScrollbarThemeAura selectively invalidate scrollbar parts.

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 226 matching lines @@
 void ScrollableArea::scrollPositionChanged(const DoublePoint& position, ScrollType scrollType)
 {
     TRACE_EVENT0("blink", "ScrollableArea::scrollPositionChanged");
 
     DoublePoint oldPosition = scrollPositionDouble();
     DoublePoint truncatedPosition = shouldUseIntegerScrollOffset() ? flooredIntPoint(position) : position;
 
     // Tell the derived class to scroll its contents.
     setScrollOffset(truncatedPosition, scrollType);
 
-    Scrollbar* verticalScrollbar = this->verticalScrollbar();
-
     // Tell the scrollbars to update their thumb postions.
+    // If a scrollbar does not have its own layer, it must always be invalidated
+    // to reflect the new thumb position, even if the theme would not otherwise
+    // require invalidation of any part.
     if (Scrollbar* horizontalScrollbar = this->horizontalScrollbar()) {
-        horizontalScrollbar->offsetDidChange();
-        if (horizontalScrollbar->isOverlayScrollbar() && !hasLayerForHorizontalScrollbar())
+        bool offsetChanged = horizontalScrollbar->offsetDidChange();
+        if (offsetChanged && !hasLayerForHorizontalScrollbar())
             setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
     }
-    if (verticalScrollbar) {
-        verticalScrollbar->offsetDidChange();
-        if (verticalScrollbar->isOverlayScrollbar() && !hasLayerForVerticalScrollbar())
+    if (Scrollbar* verticalScrollbar = this->verticalScrollbar()) {
+        bool offsetChanged = verticalScrollbar->offsetDidChange();
+        if (offsetChanged && !hasLayerForVerticalScrollbar())
             setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
     }
 
     if (scrollPositionDouble() != oldPosition) {
         // FIXME: Pass in DoubleSize. crbug.com/414283.
         scrollAnimator().notifyContentAreaScrolled(toFloatSize(scrollPositionDouble() - oldPosition));
     }
 
     scrollAnimator().setCurrentPosition(toFloatPoint(position));
 }
@@ skipping 317 matching lines @@
         std::max(0, size.height() - horizontalScrollbarHeight));
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink