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..e5adf279e1329b1937ca0125b8a254bec9035230 100644
--- a/third_party/WebKit/Source/core/style/StyleDifference.h
+++ b/third_party/WebKit/Source/core/style/StyleDifference.h
@@ -27,6 +27,7 @@ public:
     StyleDifference()
         : m_paintInvalidationType(NoPaintInvalidation)
         , m_layoutType(NoLayout)
+        , m_recomputeOverflow(false)
         , m_propertySpecificDifferences(0)
     { }
 
@@ -66,6 +67,9 @@ public:
     bool needsFullLayout() const { return m_layoutType == FullLayout; }
     void setNeedsFullLayout() { m_layoutType = FullLayout; }
 
+    bool needsRecomputeOverflow() const { return m_recomputeOverflow; }
+    void setNeedsRecomputeOverflow() { m_recomputeOverflow = true; }
+
     bool transformChanged() const { return m_propertySpecificDifferences & TransformChanged; }
     void setTransformChanged() { m_propertySpecificDifferences |= TransformChanged; }
 
@@ -98,7 +102,7 @@ private:
         FullLayout
     };
     unsigned m_layoutType : 2;
-
+    unsigned m_recomputeOverflow : 1;
     unsigned m_propertySpecificDifferences : 6;
 };