Avoid layout when list color changes

Source/core/rendering/RenderListItem.cpp

 /**
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Andrew Wellington (proton@wiretapped.net)
  *
  * 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 45 matching lines @@
 void RenderListItem::styleDidChange(StyleDifference diff, const RenderStyle* oldStyle)
 {
     RenderBlockFlow::styleDidChange(diff, oldStyle);
 
     if (style()->listStyleType() != NoneListStyle
         || (style()->listStyleImage() && !style()->listStyleImage()->errorOccurred())) {
         RefPtr<RenderStyle> newStyle = RenderStyle::create();
         // The marker always inherits from the list item, regardless of where it might end
         // up (e.g., in some deeply nested line box). See CSS3 spec.
         newStyle->inheritFrom(style());
-        if (!m_marker)
+        if (m_marker) {
+            // Reuse the current margins otherwise resetting the margins to initial values

[mstensho (USE GERRIT), 2014/10/14 19:06:03]

I don't think RenderListItem should have to know that margin-left and
margin-right are the very properties that the marker renderer modifies, so
better deal with that piece of logic in RenderListMarker.

NB! It should not be margin-left and margin-right, but margin-start and
margin-end!

Maybe we could move all the style fiddling code here into a RenderListMarker
method, that:
0. Creates a RenderStyle
1. Inherits properties from the list-item
2. Overrides what it needs to override (for now, margin-start and margin-end).
3. Sets the new style on the marker.

[Xianzhu, 2014/10/14 19:33:59]

Good catch! Though set{margin-left, margin-right} == set{margin-start,
margin-end} but missed the vertical case.

Done.
Done.

+            // would trigger unnecessary layout.
+            newStyle->setMarginLeft(m_marker->style()->marginLeft());
+            newStyle->setMarginRight(m_marker->style()->marginRight());
+        } else {
             m_marker = RenderListMarker::createAnonymous(this);
+        }
+
         m_marker->setStyle(newStyle.release());
     } else if (m_marker) {
         m_marker->destroy();
         m_marker = nullptr;
     }
 }
 
 void RenderListItem::willBeDestroyed()
 {
     if (m_marker) {
@@ skipping 444 matching lines @@
             // assume that all the following ones have too.
             // This gives us the opportunity to stop here and avoid
             // marking the same nodes again.
             break;
         }
         item->updateValue();
     }
 }
 
 } // namespace blink