Adding or changing any of box-shadow, outline, or border-image-outset does not need a layout..

third_party/WebKit/Source/core/style/BorderData.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * 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
@@ skipping 96 matching lines @@
             && m_right.visuallyEqual(o.m_right)
             && m_top.visuallyEqual(o.m_top)
             && m_bottom.visuallyEqual(o.m_bottom)
             && m_image == o.m_image
             && m_topLeft == o.m_topLeft
             && m_topRight == o.m_topRight
             && m_bottomLeft == o.m_bottomLeft
             && m_bottomRight == o.m_bottomRight;
     }
 
+    bool visualOverflowEqual(const BorderData& o) const
+    {
+        return m_image.outset() == o.m_image.outset();
+    }
+
     bool operator!=(const BorderData& o) const
     {
         return !(*this == o);
     }
 
     const BorderValue& left() const { return m_left; }
     const BorderValue& right() const { return m_right; }
     const BorderValue& top() const { return m_top; }
     const BorderValue& bottom() const { return m_bottom; }
 
@@ skipping 14 matching lines @@
 
     LengthSize m_topLeft;
     LengthSize m_topRight;
     LengthSize m_bottomLeft;
     LengthSize m_bottomRight;
 };
 
 } // namespace blink
 
 #endif // BorderData_h