[New Multicolumn] Disallow spanners inside floats and absolutely positioned boxes.

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
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         }
         if (ancestor->spannerPlaceholder()) {
             // FIXME: do we want to support nested spanners in a different way? The outer spanner
             // has already broken out from the columns to become sized by the multicol container,
             // which may be good enough for the inner spanner. But margins, borders, padding and
             // explicit widths on the outer spanner, or on any children between the outer and inner
             // spanner, will affect the width of the inner spanner this way, which might be
             // undesirable. The spec has nothing to say on the matter.
             return false; // Ignore nested spanners.
         }
+        if (ancestor->isFloatingOrOutOfFlowPositioned()) {
+            // TODO(mstensho): It could actually be nice to support this (although the usefulness is
+            // probably very limited), but currently our column balancing algorithm gets confused
+            // when a spanner is inside a float, because a float's position isn't always known until
+            // after layout. Similarly for absolutely positioned boxes.
+            return false;
+        }
         if (ancestor->isUnsplittableForPagination())
             return false;
     }
     ASSERT_NOT_REACHED();
     return false;
 }
 
 void LayoutMultiColumnFlowThread::addRegionToThread(LayoutMultiColumnSet* columnSet)
 {
     if (LayoutMultiColumnSet* nextSet = columnSet->nextSiblingMultiColumnSet()) {
@@ skipping 275 matching lines @@
 }
 
 bool LayoutMultiColumnFlowThread::isPageLogicalHeightKnown() const
 {
     if (LayoutMultiColumnSet* columnSet = lastMultiColumnSet())
         return columnSet->pageLogicalHeight();
     return false;
 }
 
 }