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

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef StyleDifference_h
 #define StyleDifference_h
 
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
 
 namespace blink {
 
 class StyleDifference {
     STACK_ALLOCATED();
 public:
     enum PropertyDifference {
         TransformChanged = 1 << 0,
         OpacityChanged = 1 << 1,
         ZIndexChanged = 1 << 2,
         FilterChanged = 1 << 3,
         BackdropFilterChanged = 1 << 4,
         // The object needs to issue paint invalidations if it is affected by text decorations or properties dependent on color (e.g., border or outline).
         TextDecorationOrColorChanged = 1 << 5,
         // If you add a value here, be sure to update the number of bits on m_propertySpecificDifferences.
+        VisualOverflowChanged = 1 << 6,
     };
 
     StyleDifference()
         : m_paintInvalidationType(NoPaintInvalidation)
         , m_layoutType(NoLayout)
         , m_propertySpecificDifferences(0)
     { }
 
     bool hasDifference() const { return m_paintInvalidationType || m_layoutType || m_propertySpecificDifferences; }
 
@@ skipping 42 matching lines @@
 
     bool filterChanged() const { return m_propertySpecificDifferences & FilterChanged; }
     void setFilterChanged() { m_propertySpecificDifferences |= FilterChanged; }
 
     bool backdropFilterChanged() const { return m_propertySpecificDifferences & BackdropFilterChanged; }
     void setBackdropFilterChanged() { m_propertySpecificDifferences |= BackdropFilterChanged; }
 
     bool textDecorationOrColorChanged() const { return m_propertySpecificDifferences & TextDecorationOrColorChanged; }
     void setTextDecorationOrColorChanged() { m_propertySpecificDifferences |= TextDecorationOrColorChanged; }
 
+    bool visualOverflowChanged() const { return m_propertySpecificDifferences & VisualOverflowChanged; }
+    void setVisualOverflowChanged() { m_propertySpecificDifferences |= VisualOverflowChanged; }

[Xianzhu, 2016/03/23 20:28:26]

I suggest to move this flag out of PropertyDifference and make it a normal
needsXXX flag (e.g. needsRecomputeOverflow).

The PropertyDifference bits are for cases that the required action is not clear
before knowing more contexts of the object.

+
 private:
     enum PaintInvalidationType {
         NoPaintInvalidation = 0,
         PaintInvalidationObject,
         PaintInvalidationLayer
     };
     unsigned m_paintInvalidationType : 2;
 
     enum LayoutType {
         NoLayout = 0,
         PositionedMovement,
         FullLayout
     };
     unsigned m_layoutType : 2;
 
-    unsigned m_propertySpecificDifferences : 6;
+    unsigned m_propertySpecificDifferences : 7;
 };
 
 } // namespace blink
 
 #endif // StyleDifference_h