All ancestor multicols must have enough rows before laying out some inner multicol.

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 160 matching lines @@
 
 LayoutMultiColumnSet* LayoutMultiColumnSet::previousSiblingMultiColumnSet() const
 {
     for (LayoutObject* sibling = previousSibling(); sibling; sibling = sibling->previousSibling()) {
         if (sibling->isLayoutMultiColumnSet())
             return toLayoutMultiColumnSet(sibling);
     }
     return nullptr;
 }
 
-bool LayoutMultiColumnSet::hasFragmentainerGroupForColumnAt(LayoutUnit bottomOffsetInFlowThread) const
+bool LayoutMultiColumnSet::hasFragmentainerGroupForColumnAt(LayoutUnit offsetInFlowThread, PageBoundaryRule pageBoundaryRule) const
 {
     const MultiColumnFragmentainerGroup& lastRow = lastFragmentainerGroup();
-    if (lastRow.logicalTopInFlowThread() > bottomOffsetInFlowThread)
-        return true;
-    return bottomOffsetInFlowThread - lastRow.logicalTopInFlowThread() <= lastRow.logicalHeight() * usedColumnCount();
+    LayoutUnit maxLogicalBottomInFlowThread = lastRow.logicalTopInFlowThread() + lastRow.logicalHeight() * usedColumnCount();
+    if (pageBoundaryRule == AssociateWithFormerPage)
+        return offsetInFlowThread <= maxLogicalBottomInFlowThread;
+    return offsetInFlowThread < maxLogicalBottomInFlowThread;
 }
 
 MultiColumnFragmentainerGroup& LayoutMultiColumnSet::appendNewFragmentainerGroup()
 {
     MultiColumnFragmentainerGroup newGroup(*this);
     { // Extra scope here for previousGroup; it's potentially invalid once we modify the m_fragmentainerGroups Vector.
         MultiColumnFragmentainerGroup& previousGroup = m_fragmentainerGroups.last();
 
         // This is the flow thread block offset where |previousGroup| ends and |newGroup| takes over.
         LayoutUnit blockOffsetInFlowThread = previousGroup.logicalTopInFlowThread() + previousGroup.logicalHeight() * usedColumnCount();
@@ skipping 255 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 } // namespace blink