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

third_party/WebKit/Source/core/layout/LayoutBlock.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 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 251 matching lines @@
 #if ENABLE(ASSERT)
     void checkPositionedObjectsNeedLayout();
 #endif
 #ifndef NDEBUG
     void showLineTreeAndMark(const InlineBox* = nullptr, const char* = nullptr, const InlineBox* = nullptr, const char* = nullptr, const LayoutObject* = nullptr) const;
 #endif
 
 protected:
     bool recalcNormalFlowChildOverflowIfNeeded(LayoutObject*);
 public:
-    bool recalcChildOverflowAfterStyleChange();
+    virtual bool recalcChildOverflowAfterStyleChange();
     bool recalcOverflowAfterStyleChange();
 
     // An example explaining layout tree structure about first-line style:
     // <style>
     //   #enclosingFirstLineStyleBlock::first-line { ... }
     // </style>
     // <div id="enclosingFirstLineStyleBlock">
     //   <div>
     //     <div id="nearestInnerBlockWithFirstLine">
     //       [<span>]first line text[</span>]
@@ skipping 235 matching lines @@
     // FIXME: This is temporary as we move code that accesses block flow
     // member variables out of LayoutBlock and into LayoutBlockFlow.
     friend class LayoutBlockFlow;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBlock, isLayoutBlock());
 
 } // namespace blink
 
 #endif // LayoutBlock_h