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

Source/core/rendering/RenderBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1846 matching lines @@
                         }
                     }
                 }
             }
         }
 
         parentBlockFlow->markAllDescendantsWithFloatsForLayout();
         parentBlockFlow->markSiblingsWithFloatsForLayout();
     }
 
-    createMultiColumnFlowThreadIfNeeded();
+    if ((diff == StyleDifferenceLayout || !oldStyle) && document().regionBasedColumnsEnabled()) {

[eseidel, 2014/04/09 17:57:51]

It may read nicer as a helper. I'm not sure the benifit of inlining this.

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

Done.

+        bool needsFlowThread = style()->specifiesColumns();
+        if (!needsFlowThread != !multiColumnFlowThread()) {

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

Please don't do this double negation with the ! on both sides like this.

needsFlowThread == static_cast<bool>(multiColumnFlowThread())

of you can add a hasMultiColumnFlowThread() method.

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

Done.

+            if (needsFlowThread) {
+                RenderMultiColumnFlowThread* flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
+                addChild(flowThread);
+                flowThread->populate();
+                RenderBlockFlowRareData& rareData = ensureRareData();
+                ASSERT(!rareData.m_multiColumnFlowThread);
+                rareData.m_multiColumnFlowThread = flowThread;

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

Can all of this live in a method instead of here?

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

Done.

+            } else {
+                multiColumnFlowThread()->evacuateAndDestroy();

[eseidel, 2014/04/09 17:57:51]

Is it typical for renderers to move their children around on style changes like
this?  I can't think of other examples.

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

I don't think we normally do this, instead we usually just reattach the subtree.

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

No, I think this is pretty special. The renderer of the DOM node which
establishes the multicol container redirects all its DOM children to an
anonymous flow thread renderer child of the multicol container.

If it ceases to be a multicol container, its flow thread is removed, and all the
flow thread's children need to be promoted. (And the other way around when a
regular block becomes a multicol container.)

+                ASSERT(!multiColumnFlowThread());
+            }
+        }
+    }
 }
 
 void RenderBlockFlow::updateStaticInlinePositionForChild(RenderBox* child, LayoutUnit logicalTop)
 {
     if (child->style()->isOriginalDisplayInlineType())
         setStaticInlinePositionForChild(child, logicalTop, startAlignedOffsetForLine(logicalTop, false));
     else
         setStaticInlinePositionForChild(child, logicalTop, startOffsetForContent());
 }
 
@@ skipping 893 matching lines @@
     bool hasStrongDirectionality;
     run.setDirection(directionForRun(run, hasStrongDirectionality));
     return run;
 }
 
 RootInlineBox* RenderBlockFlow::createRootInlineBox()
 {
     return new RootInlineBox(*this);
 }
 
-void RenderBlockFlow::createMultiColumnFlowThreadIfNeeded()
-{
-    if ((style()->hasAutoColumnCount() && style()->hasAutoColumnWidth()) || !document().regionBasedColumnsEnabled())
-        return;
-
-    if (multiColumnFlowThread())
-        return;
-
-    setChildrenInline(false);
-    RenderMultiColumnFlowThread* flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
-    RenderBlock::addChild(flowThread);
-    RenderBlockFlowRareData& rareData = ensureRareData();
-    ASSERT(!rareData.m_multiColumnFlowThread);
-    rareData.m_multiColumnFlowThread = flowThread;
-}
-
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtr(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace WebCore