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

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 64 matching lines @@
     const MultiColumnFragmentainerGroup& fragmentainerGroupAtVisualPoint(const LayoutPoint&) const;
 
     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; }
+    void propagateTallestUnbreakableLogicalHeight(LayoutUnit value) { m_tallestUnbreakableLogicalHeight = std::max(value, m_tallestUnbreakableLogicalHeight); }
 
     LayoutFlowThread* flowThread() const { return m_flowThread; }
 
     LayoutBlockFlow* multiColumnBlockFlow() const { return toLayoutBlockFlow(parent()); }
     LayoutMultiColumnFlowThread* multiColumnFlowThread() const { return toLayoutMultiColumnFlowThread(flowThread()); }
 
     LayoutMultiColumnSet* nextSiblingMultiColumnSet() const;
     LayoutMultiColumnSet* previousSiblingMultiColumnSet() const;
 
     MultiColumnFragmentainerGroup& appendNewFragmentainerGroup();
@@ skipping 66 matching lines @@
 
     void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     PositionWithAffinity positionForPoint(const LayoutPoint&) override;
 
     void paintObject(const PaintInfo&, const LayoutPoint& paintOffset) const override;
 
     void addOverflowFromChildren() override;
 
     MultiColumnFragmentainerGroupList m_fragmentainerGroups;
     LayoutFlowThread* m_flowThread;
+
+    // Height of the tallest piece of unbreakable content. This is the minimum column logical height
+    // required to avoid fragmentation where it shouldn't occur (inside unbreakable content, between
+    // orphans and widows, etc.). We only store this so that outer fragmentation contexts (if any)
+    // can query this when calculating their own minimum. Note that we don't store this value in
+    // every fragmentainer group (but rather here, in the column set), since we only need the
+    // largest one among them.
+    LayoutUnit m_tallestUnbreakableLogicalHeight;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSet, isLayoutMultiColumnSet());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnSet_h