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

Source/core/rendering/RenderMultiColumnFlowThread.cpp

 /*
  * Copyright (C) 2012 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 34 matching lines @@
 }
 
 RenderMultiColumnFlowThread* RenderMultiColumnFlowThread::createAnonymous(Document& document, RenderStyle* parentStyle)
 {
     RenderMultiColumnFlowThread* renderer = new RenderMultiColumnFlowThread();
     renderer->setDocumentForAnonymous(&document);
     renderer->setStyle(RenderStyle::createAnonymousStyleWithDisplay(parentStyle, BLOCK));
     return renderer;
 }
 
+void RenderMultiColumnFlowThread::populate()
+{
+    RenderBlockFlow* multicolContainer = multiColumnBlockFlow();
+    ASSERT(!nextSibling());
+    // Reparent children preceding the flow thread into the flow thread. It's multicol content
+    // now. At this point there's obviously nothing after the flow thread, but renderers (column
+    // sets and spanners) will be inserted there as we insert elements into the flow thread.
+    multicolContainer->moveChildrenTo(this, multicolContainer->firstChild(), this, true);
+}
+
+void RenderMultiColumnFlowThread::evacuateAndDestroy()
+{
+    RenderBlockFlow* multicolContainer = multiColumnBlockFlow();
+
+    // Remove all sets.
+    for (RenderBox* sibling = nextSiblingBox(); sibling;) {
+        RenderBox* nextSibling = sibling->nextSiblingBox();
+        if (sibling->isRenderMultiColumnSet())
+            sibling->destroy();
+        sibling = nextSibling;
+    }
+
+    ASSERT(!previousSibling());
+    ASSERT(!nextSibling());
+
+    // Finally we can promote all flow thread's children. Before we move them to the flow thread's
+    // container, we need to unregister the flow thread, so that they aren't just re-added again to
+    // the flow thread that we're trying to empty.
+    multicolContainer->resetMultiColumnFlowThread();
+    moveAllChildrenTo(multicolContainer, true);
+
+    destroy();
+}
+
 void RenderMultiColumnFlowThread::layoutColumns(bool relayoutChildren, SubtreeLayoutScope& layoutScope)
 {
     // Update the dimensions of our regions before we lay out the flow thread.
     // FIXME: Eventually this is going to get way more complicated, and we will be destroying regions
     // instead of trying to keep them around.
     RenderBlockFlow* container = multiColumnBlockFlow();
     bool shouldInvalidateRegions = false;
     for (RenderBox* childBox = container->firstChildBox(); childBox; childBox = childBox->nextSiblingBox()) {
         if (childBox == this)
             continue;
@@ skipping 168 matching lines @@
 {
     for (RenderBox* renderer = parentBox()->lastChildBox(); renderer; renderer = renderer->previousSiblingBox()) {
         if (renderer->isRenderMultiColumnSet())
             return toRenderMultiColumnSet(renderer)->computedColumnHeight();
     }
     // A column set hasn't been created yet. Height may already be known if column-fill is 'auto', though.
     return !requiresBalancing();
 }
 
 }