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

third_party/WebKit/Source/core/style/ComputedStyle.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. 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 517 matching lines @@
             return true;
 
         if (rareNonInheritedData->m_deprecatedFlexibleBox.get() != other.rareNonInheritedData->m_deprecatedFlexibleBox.get()
             && *rareNonInheritedData->m_deprecatedFlexibleBox.get() != *other.rareNonInheritedData->m_deprecatedFlexibleBox.get())
             return true;
 
         if (rareNonInheritedData->m_flexibleBox.get() != other.rareNonInheritedData->m_flexibleBox.get()
             && *rareNonInheritedData->m_flexibleBox.get() != *other.rareNonInheritedData->m_flexibleBox.get())
             return true;
 
-        // FIXME: We should add an optimized form of layout that just recomputes visual overflow.
-        if (!rareNonInheritedData->shadowDataEquivalent(*other.rareNonInheritedData.get()))
-            return true;
-
         if (!rareNonInheritedData->reflectionDataEquivalent(*other.rareNonInheritedData.get()))
             return true;
 
         if (rareNonInheritedData->m_multiCol.get() != other.rareNonInheritedData->m_multiCol.get()
             && *rareNonInheritedData->m_multiCol.get() != *other.rareNonInheritedData->m_multiCol.get())
             return true;
 
         // If the counter directives change, trigger a relayout to re-calculate counter values and rebuild the counter node tree.
         const CounterDirectiveMap* mapA = rareNonInheritedData->m_counterDirectives.get();
         const CounterDirectiveMap* mapB = other.rareNonInheritedData->m_counterDirectives.get();
@@ skipping 100 matching lines @@
             return true;
     } else if (noninherited_flags.effectiveDisplay == LIST_ITEM) {
         if (inherited_flags._list_style_type != other.inherited_flags._list_style_type
             || inherited_flags._list_style_position != other.inherited_flags._list_style_position)
             return true;
     }
 
     if ((visibility() == COLLAPSE) != (other.visibility() == COLLAPSE))
         return true;
 
-    if (!m_background->outline().visuallyEqual(other.m_background->outline())) {
-        // FIXME: We only really need to recompute the overflow but we don't have an optimized layout for it.
-        return true;
-    }
-
     if (hasPseudoStyle(PseudoIdScrollbar) != other.hasPseudoStyle(PseudoIdScrollbar))
         return true;
 
     // Movement of non-static-positioned object is special cased in ComputedStyle::visualInvalidationDiff().
 
     return false;
 }
 
 bool ComputedStyle::diffNeedsFullLayout(const ComputedStyle& other) const
 {
@@ skipping 49 matching lines @@
         if (rareNonInheritedData->m_mask != other.rareNonInheritedData->m_mask
             || rareNonInheritedData->m_maskBoxImage != other.rareNonInheritedData->m_maskBoxImage)
             return true;
     }
 
     return false;
 }
 
 bool ComputedStyle::diffNeedsPaintInvalidationObject(const ComputedStyle& other) const
 {
+    if (!m_background->outline().visuallyEqual(other.m_background->outline()))
+        return true;

[Xianzhu, 2016/03/22 17:02:05]

We can avoid this check by adjusting visualOverflowChanged to
needsPaintInvalidationObject in adjustStyleDifference in LayoutObject.cpp.

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

Please ignore this comment if you take the
StyleDifference::needsRecomputeOverflow() way.

     if (inherited_flags._visibility != other.inherited_flags._visibility
         || inherited_flags.m_printColorAdjust != other.inherited_flags.m_printColorAdjust
         || inherited_flags._insideLink != other.inherited_flags._insideLink
         || !surround->border.visuallyEqual(other.surround->border)
         || !m_background->visuallyEqual(*other.m_background))
         return true;
 
     if (rareInheritedData.get() != other.rareInheritedData.get()) {
         if (rareInheritedData->userModify != other.rareInheritedData->userModify
             || rareInheritedData->userSelect != other.rareInheritedData->userSelect
             || rareInheritedData->m_imageRendering != other.rareInheritedData->m_imageRendering)
             return true;
     }
 
     if (rareNonInheritedData.get() != other.rareNonInheritedData.get()) {
         if (rareNonInheritedData->userDrag != other.rareNonInheritedData->userDrag
             || rareNonInheritedData->m_objectFit != other.rareNonInheritedData->m_objectFit
             || rareNonInheritedData->m_objectPosition != other.rareNonInheritedData->m_objectPosition
+            || !rareNonInheritedData->shadowDataEquivalent(*other.rareNonInheritedData.get())

[Xianzhu, 2016/03/22 17:02:05]

Ditto.

             || !rareNonInheritedData->shapeOutsideDataEquivalent(*other.rareNonInheritedData.get())
             || !rareNonInheritedData->clipPathDataEquivalent(*other.rareNonInheritedData.get())
             || (visitedLinkBorderLeftColor() != other.visitedLinkBorderLeftColor() && borderLeftWidth())
             || (visitedLinkBorderRightColor() != other.visitedLinkBorderRightColor() && borderRightWidth())
             || (visitedLinkBorderBottomColor() != other.visitedLinkBorderBottomColor() && borderBottomWidth())
             || (visitedLinkBorderTopColor() != other.visitedLinkBorderTopColor() && borderTopWidth())
             || (visitedLinkOutlineColor() != other.visitedLinkOutlineColor() && outlineWidth())
             || (visitedLinkBackgroundColor() != other.visitedLinkBackgroundColor()))
             return true;
     }
@@ skipping 13 matching lines @@
     if (rareNonInheritedData.get() != other.rareNonInheritedData.get()) {
         if (!transformDataEquivalent(other))
             diff.setTransformChanged();
 
         if (rareNonInheritedData->opacity != other.rareNonInheritedData->opacity)
             diff.setOpacityChanged();
 
         if (rareNonInheritedData->m_filter != other.rareNonInheritedData->m_filter)
             diff.setFilterChanged();
 
+        if (!rareNonInheritedData->shadowDataEquivalent(*other.rareNonInheritedData.get()))
+            diff.setVisualOverflowChanged();

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

Move this to diffNeedsRecomputeOverflow() if you take the
StyleDifference::needsRecomputeOverflow() way.

+
         if (rareNonInheritedData->m_backdropFilter != other.rareNonInheritedData->m_backdropFilter)
             diff.setBackdropFilterChanged();
     }
 
+    if (!m_background->outline().visuallyEqual(other.m_background->outline()) || !surround->border.visualOverflowEqual(other.surround->border))
+        diff.setVisualOverflowChanged();

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

Ditto.

+
     if (!diff.needsPaintInvalidation()) {
         if (inherited->color != other.inherited->color
             || inherited->visitedLinkColor != other.inherited->visitedLinkColor
             || inherited_flags.m_textUnderline != other.inherited_flags.m_textUnderline
             || visual->textDecoration != other.visual->textDecoration) {
             diff.setTextDecorationOrColorChanged();
         } else if (rareNonInheritedData.get() != other.rareNonInheritedData.get()
             && (rareNonInheritedData->m_textDecorationStyle != other.rareNonInheritedData->m_textDecorationStyle
                 || rareNonInheritedData->m_textDecorationColor != other.rareNonInheritedData->m_textDecorationColor
                 || rareNonInheritedData->m_visitedLinkTextDecorationColor != other.rareNonInheritedData->m_visitedLinkTextDecorationColor)) {
@@ skipping 1080 matching lines @@
     if (!shadowList)
         return false;
     for (size_t i = shadowList->shadows().size(); i--; ) {
         if (shadowList->shadows()[i].color().isCurrentColor())
             return true;
     }
     return false;
 }
 
 } // namespace blink