Look inside inner nested multicols to calculate minimum space shortage.

third_party/WebKit/Source/core/layout/ColumnBalancer.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "core/layout/ColumnBalancer.h"
 
+#include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/LayoutMultiColumnSet.h"
 
 namespace blink {
 
 ColumnBalancer::ColumnBalancer(const MultiColumnFragmentainerGroup& group)
     : m_group(group)
 {
 }
 
 void ColumnBalancer::traverse()
@@ skipping 247 matching lines @@
         if (isFirstAfterBreak || group().columnLogicalTopForOffset(flowThreadOffset()) != group().columnLogicalTopForOffset(bottomInFlowThread)) {
             // If the child crosses a column boundary, record space shortage, in case nothing
             // inside it has already done so. The column balancer needs to know by how much it
             // has to stretch the columns to make more content fit. If no breaks are reported
             // (but do occur), the balancer will have no clue. Only measure the space after the
             // last column boundary, in case it crosses more than one.
             LayoutUnit spaceUsedInLastColumn = bottomInFlowThread - group().columnLogicalTopForOffset(bottomInFlowThread);
             recordSpaceShortage(spaceUsedInLastColumn);
         }
     }
+
+    // If this is an inner multicol container, look for space shortage inside it.
+    if (!box.isLayoutBlockFlow())
+        return;
+    LayoutMultiColumnFlowThread* flowThread = toLayoutBlockFlow(box).multiColumnFlowThread();
+    if (!flowThread)
+        return;
+    for (const LayoutMultiColumnSet* columnSet = flowThread->firstMultiColumnSet(); columnSet; columnSet = columnSet->nextSiblingMultiColumnSet()) {
+        for (const MultiColumnFragmentainerGroup& row : columnSet->fragmentainerGroups()) {
+            MinimumSpaceShortageFinder innerFinder(row);
+            recordSpaceShortage(innerFinder.minimumSpaceShortage());
+        }
+    }
 }
 
 void MinimumSpaceShortageFinder::examineBoxBeforeLeaving(const LayoutBox& box)
 {
     if (m_pendingStrut == LayoutUnit::min() || box.paginationBreakability() != LayoutBox::ForbidBreaks)
         return;
 
     // The previous break was before a breakable block. Here's the first piece of unbreakable
     // content after / inside that block. We want to record the distance from the top of the column
     // to the bottom of this box as space shortage.
@@ skipping 15 matching lines @@
         recordSpaceShortage(logicalOffsetFromCurrentColumn + lineHeight - m_pendingStrut);
         m_pendingStrut = LayoutUnit::min();
         return;
     }
     ASSERT(isFirstAfterBreak(lineTopInFlowThread) || !line.paginationStrut());
     if (isFirstAfterBreak(lineTopInFlowThread))
         recordSpaceShortage(lineHeight - line.paginationStrut());
 }
 
 } // namespace blink