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.
     };
 
     StyleDifference()
         : m_paintInvalidationType(NoPaintInvalidation)
         , m_layoutType(NoLayout)
+        , m_recomputeOverflow(NoRecomputeFlow)
         , m_propertySpecificDifferences(0)
     { }
 
     bool hasDifference() const { return m_paintInvalidationType || m_layoutType || m_propertySpecificDifferences; }
 
     bool hasAtMostPropertySpecificDifferences(unsigned propertyDifferences) const
     {
         return !m_paintInvalidationType && !m_layoutType && !(m_propertySpecificDifferences & ~propertyDifferences);
     }
 
@@ skipping 19 matching lines @@
     bool needsPositionedMovementLayout() const { return m_layoutType == PositionedMovement; }
     void setNeedsPositionedMovementLayout()
     {
         ASSERT(!needsFullLayout());
         m_layoutType = PositionedMovement;
     }
 
     bool needsFullLayout() const { return m_layoutType == FullLayout; }
     void setNeedsFullLayout() { m_layoutType = FullLayout; }
 
+    bool needsRecomputeOverflow() const { return m_recomputeOverflow == RecomputeOverflow; }
+    void setNeedsRecomputeOverflow() { m_recomputeOverflow = RecomputeOverflow; }
+
     bool transformChanged() const { return m_propertySpecificDifferences & TransformChanged; }
     void setTransformChanged() { m_propertySpecificDifferences |= TransformChanged; }
 
     bool opacityChanged() const { return m_propertySpecificDifferences & OpacityChanged; }
     void setOpacityChanged() { m_propertySpecificDifferences |= OpacityChanged; }
 
     bool zIndexChanged() const { return m_propertySpecificDifferences & ZIndexChanged; }
     void setZIndexChanged() { m_propertySpecificDifferences |= ZIndexChanged; }
 
     bool filterChanged() const { return m_propertySpecificDifferences & FilterChanged; }
@@ skipping 13 matching lines @@
     };
     unsigned m_paintInvalidationType : 2;
 
     enum LayoutType {
         NoLayout = 0,
         PositionedMovement,
         FullLayout
     };
     unsigned m_layoutType : 2;
 
+    enum RecomputeOverflowType {

[Xianzhu, 2016/04/15 04:59:16]

We don't need to define a private enum to replace a bool like this. Using bool
in the getter and setter is already clear enough.

(We do prefer enum to bool for function parameters where the meaning of
true/false is not clear at call sites.)

[rhogan, 2016/04/15 19:34:32]

Sounds good.

+        NoRecomputeFlow = 0,
+        RecomputeOverflow
+    };
+    unsigned m_recomputeOverflow : 1;
+
     unsigned m_propertySpecificDifferences : 6;
 };
 
 } // namespace blink
 
 #endif // StyleDifference_h