overflow-y: hidden areas can still be scrolled vertically

Source/core/rendering/RenderBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 446 matching lines @@
     bool autoscrollInProgress() const;
     bool canAutoscroll() const;
     IntSize calculateAutoscrollDirection(const IntPoint& windowPoint) const;
     static RenderBox* findAutoscrollable(RenderObject*);
     virtual void stopAutoscroll() { }
     virtual void panScroll(const IntPoint&);
 
     bool hasAutoVerticalScrollbar() const { return hasOverflowClip() && (style()->overflowY() == OAUTO || style()->overflowY() == OOVERLAY); }
     bool hasAutoHorizontalScrollbar() const { return hasOverflowClip() && (style()->overflowX() == OAUTO || style()->overflowX() == OOVERLAY); }
     bool scrollsOverflow() const { return scrollsOverflowX() || scrollsOverflowY(); }
+
+    bool hasScrollableOverflowX() const { return scrollsOverflowX() && scrollWidth() != clientWidth(); }
+    bool hasScrollableOverflowY() const { return scrollsOverflowY() && scrollHeight() != clientHeight(); }
     virtual bool scrollsOverflowX() const { return hasOverflowClip() && (style()->overflowX() == OSCROLL || hasAutoHorizontalScrollbar()); }
     virtual bool scrollsOverflowY() const { return hasOverflowClip() && (style()->overflowY() == OSCROLL || hasAutoVerticalScrollbar()); }
     bool usesCompositedScrolling() const;
 
     // Elements such as the <input> field override this to specify that they are scrollable
     // outside the context of the CSS overflow style
     virtual bool isIntristicallyScrollable(ScrollbarOrientation orientation) const { return false; }
 
     bool hasUnsplittableScrollingOverflow() const;
     bool isUnsplittableForPagination() const;
@@ skipping 281 matching lines @@
         if (UNLIKELY(m_inlineBoxWrapper != 0))
             deleteLineBoxWrapper();
     }
 
     m_inlineBoxWrapper = boxWrapper;
 }
 
 } // namespace WebCore
 
 #endif // RenderBox_h