When balancing columns, we must check inner multicols for unbreakable content.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.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 @@
     return row.visualPointToFlowThreadPoint(visualPoint - row.offsetFromColumnSet());
 }
 
 LayoutUnit LayoutMultiColumnSet::pageLogicalTopForOffset(LayoutUnit offset) const
 {
     return fragmentainerGroupAtFlowThreadOffset(offset).columnLogicalTopForOffset(offset);
 }
 
 bool LayoutMultiColumnSet::recalculateColumnHeight(BalancedColumnHeightCalculation calculationMode)
 {
+    if (calculationMode == GuessFromFlowThreadPortion)
+        m_tallestUnbreakableLogicalHeight = LayoutUnit();
     bool changed = false;
     for (auto& group : m_fragmentainerGroups)
         changed = group.recalculateColumnHeight(calculationMode) || changed;
     return changed;
 }
 
 void LayoutMultiColumnSet::resetColumnHeight()
 {
     m_fragmentainerGroups.deleteExtraGroups();
     m_fragmentainerGroups.first().resetColumnHeight();
@@ skipping 119 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 }