Mouse up updates scrollbar hovered part if needed.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayMock.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayMock.h b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayMock.h
index d7877208a9506bb034b7dc5c4ac2a93f7eb48265..1022c2b2b1c47c73126368059e7c2f22428339ac 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayMock.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayMock.h
@@ -58,6 +58,11 @@ class PLATFORM_EXPORT ScrollbarThemeOverlayMock : public ScrollbarThemeOverlay {
     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; }