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

third_party/WebKit/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 244 matching lines @@
     multicolContainer->resetMultiColumnFlowThread();
     moveAllChildrenTo(multicolContainer, true);
 
     // We used to manually nuke the line box tree here, but that should happen automatically when
     // moving children around (the code above).
     ASSERT(!firstLineBox());
 
     destroy();
 }
 
+LayoutUnit LayoutMultiColumnFlowThread::tallestUnbreakableLogicalHeight(LayoutUnit offsetInFlowThread) const
+{
+    if (LayoutMultiColumnSet* multicolSet = columnSetAtBlockOffset(offsetInFlowThread))
+        return multicolSet->tallestUnbreakableLogicalHeight();
+    return LayoutUnit();
+}
+
 LayoutSize LayoutMultiColumnFlowThread::columnOffset(const LayoutPoint& point) const
 {
     if (!hasValidColumnSetInfo())
         return LayoutSize(0, 0);
 
     LayoutPoint flowThreadPoint = flipForWritingMode(point);
     LayoutUnit blockOffset = isHorizontalWritingMode() ? flowThreadPoint.y() : flowThreadPoint.x();
     return flowThreadTranslationAtOffset(blockOffset);
 }
 
@@ skipping 647 matching lines @@
     LayoutMultiColumnSet* columnSet = columnSetAtBlockOffset(logicalTopInFlowThreadAfterPagination);
     if (!columnSet)
         return;
     MultiColumnFragmentainerGroup& row = columnSet->fragmentainerGroupAtFlowThreadOffset(logicalTopInFlowThreadAfterPagination);
     if (!row.isLastGroup())
         return;
     appendNewFragmentainerGroupIfNeeded(logicalTopInFlowThreadAfterPagination);
 }
 
 }