Look inside inner nested multicols to calculate minimum space shortage.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.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 55 matching lines @@
     const MultiColumnFragmentainerGroup& lastFragmentainerGroup() const { return m_fragmentainerGroups.last(); }
     MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit flowThreadOffset)
     {
         return m_fragmentainerGroups[fragmentainerGroupIndexAtFlowThreadOffset(flowThreadOffset)];
     }
     const MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit flowThreadOffset) const
     {
         return m_fragmentainerGroups[fragmentainerGroupIndexAtFlowThreadOffset(flowThreadOffset)];
     }
     const MultiColumnFragmentainerGroup& fragmentainerGroupAtVisualPoint(const LayoutPoint&) const;
+    const MultiColumnFragmentainerGroupList& fragmentainerGroups() const { return m_fragmentainerGroups; }
 
     bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutMultiColumnSet || LayoutBlockFlow::isOfType(type); }
     bool canHaveChildren() const final { return false; }
 
     // Return the width and height of a single column or page in the set.
     LayoutUnit pageLogicalWidth() const { return flowThread()->logicalWidth(); }
     LayoutUnit pageLogicalHeightForOffset(LayoutUnit) const;
     LayoutUnit pageRemainingLogicalHeightForOffset(LayoutUnit, PageBoundaryRule) const;
     bool isPageLogicalHeightKnown() const;
     LayoutUnit tallestUnbreakableLogicalHeight() const { return m_tallestUnbreakableLogicalHeight; }
@@ skipping 93 matching lines @@
     // largest one among them.
     LayoutUnit m_tallestUnbreakableLogicalHeight;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSet, isLayoutMultiColumnSet());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnSet_h