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

Index: third_party/WebKit/Source/core/style/StyleDifference.h
diff --git a/third_party/WebKit/Source/core/style/StyleDifference.h b/third_party/WebKit/Source/core/style/StyleDifference.h
index 360642f7a45780e0f369111b07e80fcb8d5ee213..c5447d2e48ac9d1073f1340aa371f876bdcb51f7 100644
--- a/third_party/WebKit/Source/core/style/StyleDifference.h
+++ b/third_party/WebKit/Source/core/style/StyleDifference.h
@@ -22,6 +22,7 @@ public:
         // 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()
@@ -84,6 +85,9 @@ public:
     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,
@@ -99,7 +103,7 @@ private:
     };
     unsigned m_layoutType : 2;
 
-    unsigned m_propertySpecificDifferences : 6;
+    unsigned m_propertySpecificDifferences : 7;
 };
 
 } // namespace blink