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

third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.h

 /*
  * 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 144 matching lines @@
     // Empty the flow thread by moving everything to the parent. Remove all multicol specific
     // layoutObjects. Then destroy the flow thread. Called when a multicol container becomes a regular
     // block.
     void evacuateAndDestroy();
 
     unsigned columnCount() const { return m_columnCount; }
     LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
     void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
     bool progressionIsInline() const { return m_progressionIsInline; }
 
+    LayoutUnit tallestUnbreakableLogicalHeight(LayoutUnit offsetInFlowThread) const;
+
     LayoutSize columnOffset(const LayoutPoint&) const final;
 
     // Do we need to set a new width and lay out?
     virtual bool needsNewWidth() const;
 
     bool isPageLogicalHeightKnown() const final;
 
     LayoutSize flowThreadTranslationAtOffset(LayoutUnit) const;
 
     LayoutPoint visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const override;
@@ skipping 71 matching lines @@
     bool m_isBeingEvacuated;
 };
 
 // Cannot use DEFINE_LAYOUT_OBJECT_TYPE_CASTS here, because isMultiColumnFlowThread() is defined in
 // LayoutFlowThread, not in LayoutObject.
 DEFINE_TYPE_CASTS(LayoutMultiColumnFlowThread, LayoutFlowThread, object, object->isLayoutMultiColumnFlowThread(), object.isLayoutMultiColumnFlowThread());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnFlowThread_h