Make ScrollbarThemeAura selectively invalidate scrollbar parts.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm

 /*
  * Copyright (C) 2013 Google 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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 86 matching lines @@
     return *map;
 }
 
 static bool supportsExpandedScrollbars()
 {
     // FIXME: This is temporary until all platforms that support ScrollbarPainter support this part of the API.
     static bool globalSupportsExpandedScrollbars = [NSClassFromString(@"NSScrollerImp") instancesRespondToSelector:@selector(setExpanded:)];
     return globalSupportsExpandedScrollbars;
 }
 
+ScrollbarPart ScrollbarThemeMacOverlayAPI::invalidateOnThumbPositionChange(
+    const ScrollbarThemeClient& scrollbar, float oldPosition, float newPosition) const
+{
+    // ScrollAnimatorMac will invalidate scrollbar parts if necessary.
+    return NoPart;
+}
+
 void ScrollbarThemeMacOverlayAPI::registerScrollbar(ScrollbarThemeClient& scrollbar)
 {
     ScrollbarThemeMacCommon::registerScrollbar(scrollbar);
 
     bool isHorizontal = scrollbar.orientation() == HorizontalScrollbar;
     ScrollbarPainter scrollbarPainter = [NSClassFromString(@"NSScrollerImp") scrollerImpWithStyle:recommendedScrollerStyle() controlSize:(NSControlSize)scrollbar.controlSize() horizontal:isHorizontal replacingScrollerImp:nil];
     RetainPtr<BlinkScrollbarObserver> observer = [[BlinkScrollbarObserver alloc] initWithScrollbar:&scrollbar painter:scrollbarPainter];
 
     scrollbarPainterMap().add(&scrollbar, observer);
     updateEnabledState(scrollbar);
@@ skipping 152 matching lines @@
     [painterForScrollbar(scrollbar) setEnabled:scrollbar.enabled()];
 }
 
 float ScrollbarThemeMacOverlayAPI::thumbOpacity(const ScrollbarThemeClient& scrollbar) const {
     ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
     return [scrollbarPainter knobAlpha];
 }
 
 } // namespace blink