Changing between multicol and regular block shouldn't recreate all renderers.

Source/core/rendering/style/RenderStyle.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 161 matching lines @@
 StyleRecalcChange RenderStyle::stylePropagationDiff(const RenderStyle* oldStyle, const RenderStyle* newStyle)
 {
     if ((!oldStyle && newStyle) || (oldStyle && !newStyle))
         return Reattach;
 
     if (!oldStyle && !newStyle)
         return NoChange;
 
     if (oldStyle->display() != newStyle->display()
         || oldStyle->hasPseudoStyle(FIRST_LETTER) != newStyle->hasPseudoStyle(FIRST_LETTER)
         || oldStyle->columnSpan() != newStyle->columnSpan()

[esprehn, 2014/04/09 18:19:08]

We still need to reattach if the span changes though? What's the benefit of
avoiding a reattach when going between multi column and not.

It's not clear what you gain from this patch. Do people really dynamically add
multi column?

[mstensho (USE GERRIT), 2014/04/09 18:58:27]

In the old implementation, yes. In the new implementation, it may be feasible to
avoid it, but I'll put that on hold until the old implementation is gone.
It's probably not very common, but if we use the multicol implementation to do
paged overflow on the viewport (overflow:-webkit-paged-x on BODY, for instance),
it should be better to do this rather than reattaching RenderView (which, last
time I checked, seemed like a problematic thing to do).

-        || oldStyle->specifiesColumns() != newStyle->specifiesColumns()

[esprehn, 2014/04/09 18:19:08]

I assume we check this bit somewhere else too?

[mstensho (USE GERRIT), 2014/04/09 18:58:27]

Yes, we do.

         || !oldStyle->contentDataEquivalent(newStyle)
         || oldStyle->hasTextCombine() != newStyle->hasTextCombine())
         return Reattach;
 
     if (*oldStyle == *newStyle)
         return diffPseudoStyles(oldStyle, newStyle);
 
     if (oldStyle->inheritedNotEqual(newStyle)
         || oldStyle->hasExplicitlyInheritedProperties()
         || newStyle->hasExplicitlyInheritedProperties())
@@ skipping 1460 matching lines @@
     // right
     radiiSum = radii.topRight().height() + radii.bottomRight().height();
     if (radiiSum > rect.height())
         factor = std::min(rect.height() / radiiSum, factor);
 
     ASSERT(factor <= 1);
     return factor;
 }
 
 } // namespace WebCore