Don't eat margins when column height isn't yet known.

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 198 matching lines @@
     for (const LayoutMultiColumnSet* candidate = firstMultiColumnSet(); candidate; candidate = candidate->nextSiblingMultiColumnSet()) {
         columnSet = candidate;
         if (candidate->logicalBottom() > blockOffset)
             break;
     }
     return columnSet ? columnSet->visualPointToFlowThreadPoint(toLayoutPoint(visualPoint + location() - columnSet->location())) : visualPoint;
 }
 
 LayoutMultiColumnSet* LayoutMultiColumnFlowThread::columnSetAtBlockOffset(LayoutUnit offset) const
 {
-    if (m_lastSetWorkedOn) {
+    if (LayoutMultiColumnSet* columnSet = m_lastSetWorkedOn) {
         // Layout in progress. We are calculating the set heights as we speak, so the column set range
         // information is not up-to-date.
-        return m_lastSetWorkedOn;
+        while (columnSet->logicalTopInFlowThread() > offset) {
+            // Sometimes we have to use a previous set. This happens when we're working with a block
+            // that contains a spanner (so that there's a column set both before and after the
+            // spanner, and both sets contain said block).
+            LayoutMultiColumnSet* previousSet = columnSet->previousSiblingMultiColumnSet();
+            if (!previousSet)
+                break;
+            columnSet = previousSet;
+        }
+        return columnSet;
     }
 
     ASSERT(!m_columnSetsInvalidated);
     if (m_multiColumnSetList.isEmpty())
         return 0;
     if (offset <= 0)
         return m_multiColumnSetList.first();
 
     MultiColumnSetSearchAdapter adapter(offset);
     m_multiColumnSetIntervalTree.allOverlapsWithAdapter<MultiColumnSetSearchAdapter>(adapter);
@@ skipping 511 matching lines @@
 }
 
 bool LayoutMultiColumnFlowThread::isPageLogicalHeightKnown() const
 {
     if (LayoutMultiColumnSet* columnSet = lastMultiColumnSet())
         return columnSet->pageLogicalHeight();
     return false;
 }
 
 }