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)
+        createOrDestroyMultiColumnFlowThreadIfNeeded();
 }
 
 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()
+void RenderBlockFlow::createOrDestroyMultiColumnFlowThreadIfNeeded()
 {
-    if ((style()->hasAutoColumnCount() && style()->hasAutoColumnWidth()) || !document().regionBasedColumnsEnabled())
+    if (!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;
+    bool needsFlowThread = style()->specifiesColumns();
+    if (needsFlowThread != static_cast<bool>(multiColumnFlowThread())) {
+        if (needsFlowThread) {
+            RenderMultiColumnFlowThread* flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
+            addChild(flowThread);
+            flowThread->populate();
+            RenderBlockFlowRareData& rareData = ensureRareData();
+            ASSERT(!rareData.m_multiColumnFlowThread);
+            rareData.m_multiColumnFlowThread = flowThread;
+        } else {
+            multiColumnFlowThread()->evacuateAndDestroy();
+            ASSERT(!multiColumnFlowThread());
+        }
+    }
 }
 
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtr(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace WebCore