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

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index 8e86cd9c3a371df555fd7d5b3ae7a4a0cf41af42..f8d99740d3e7c1f6f28f6ac58a593bfec059e9fa 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -1864,7 +1864,22 @@ void RenderBlockFlow::styleDidChange(StyleDifference diff, const RenderStyle* ol
         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)
@@ -2778,22 +2793,6 @@ 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)