Mouse up updates scrollbar hovered part if needed.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayMock.h

 /*
  * 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 40 matching lines @@
   }
 
   void paintThumb(GraphicsContext& gc,
                   const Scrollbar& scrollbar,
                   const IntRect& rect) override {
     if (!scrollbar.enabled())
       return;
     ScrollbarThemeOverlay::paintThumb(gc, scrollbar, rect);
   }
 
+  bool shouldSnapBackToDragOrigin(const ScrollbarThemeClient& scrollbar,
+                                  const PlatformMouseEvent& evt) override {
+    return false;

[bokan, 2016/12/19 18:23:50]

This is fine, but was it a problem to implement Platform or the testing subclass
of it to return a default a problem?

[sahel, 2016/12/19 19:05:53]

These unittests don't use testingPlatformSupport.

+  }
+
  private:
   double m_delayInSeconds;
   bool isMockTheme() const final { return true; }
 };
 
 }  // namespace blink
 #endif  // ScrollbarThemeOverlayMock_h