[New Multicolumn] Out-of-flow objects don't establish column sets.

Source/core/layout/LayoutMultiColumnFlowThread.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 514 matching lines @@
                     }
                 }
                 ASSERT(setToSplit || insertBefore);
             }
             createAndInsertSpannerPlaceholder(toLayoutBox(layoutObject), insertBefore);
             if (setToSplit)
                 createAndInsertMultiColumnSet(insertBefore);
             continue;
         }
         // This layoutObject is regular column content (i.e. not a spanner). Create a set if necessary.
+        while (objectAfterSubtree) {
+            // Walk through the siblings and find the first one which is either in-flow or has this
+            // flow thread as its containing block flow thread.
+            if (!objectAfterSubtree->isOutOfFlowPositioned()) {
+                // In-flow objects are always part of the flow thread (unless it's a spanner - but
+                // we'll deal with that further below).
+                break;
+            }
+            if (objectAfterSubtree->containingBlock()->flowThreadContainingBlock() == this) {
+                // This out-of-flow object is still part of the flow thread, because its containing
+                // block (probably relatively positioned) is part of the flow thread.
+                break;
+            }
+            objectAfterSubtree = objectAfterSubtree->nextInPreOrderAfterChildren(this);
+        }
         if (objectAfterSubtree) {
             if (LayoutMultiColumnSpannerPlaceholder* placeholder = objectAfterSubtree->spannerPlaceholder()) {
                 // If inserted right before a spanner, we need to make sure that there's a set for us there.
                 LayoutBox* previous = placeholder->previousSiblingMultiColumnBox();
                 if (!previous || !previous->isLayoutMultiColumnSet())
                     createAndInsertMultiColumnSet(placeholder);
             } else {
                 // Otherwise, since |objectAfterSubtree| isn't a spanner, it has to mean that there's
                 // already a set for that content. We can use it for this layoutObject too.
                 ASSERT(findSetRendering(objectAfterSubtree));
@@ skipping 170 matching lines @@
 }
 
 bool LayoutMultiColumnFlowThread::isPageLogicalHeightKnown() const
 {
     if (LayoutMultiColumnSet* columnSet = lastMultiColumnSet())
         return columnSet->pageLogicalHeight();
     return false;
 }
 
 }