Paginated containers are opaque to enclosing fragmentation contexts.

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"
@@ skipping 96 matching lines @@
     }
 
     if (box.hasForcedBreakAfter())
         addContentRun(flowThreadOffset() + box.logicalHeight());
 
     if (box.paginationBreakability() != LayoutBox::AllowAnyBreaks) {
         LayoutUnit unsplittableLogicalHeight = box.logicalHeight();
         if (box.isFloating())
             unsplittableLogicalHeight += box.marginBefore() + box.marginAfter();
         m_tallestUnbreakableLogicalHeight = std::max(m_tallestUnbreakableLogicalHeight, unsplittableLogicalHeight);
-    } else if (box.isLayoutBlockFlow()) {
-        if (LayoutMultiColumnFlowThread* innerFlowThread = toLayoutBlockFlow(box).multiColumnFlowThread()) {
-            LayoutUnit offsetInInnerFlowThread = flowThreadOffset() - innerFlowThread->blockOffsetInEnclosingFragmentationContext();
-            LayoutUnit innerUnbreakableHeight = innerFlowThread->tallestUnbreakableLogicalHeight(offsetInInnerFlowThread);
-            m_tallestUnbreakableLogicalHeight = std::max(m_tallestUnbreakableLogicalHeight, innerUnbreakableHeight);
-        }
+        return;
     }
+    // Need to examine inner multicol containers to find their tallest unbreakable piece of content.
+    if (!box.isLayoutBlockFlow())
+        return;
+    LayoutMultiColumnFlowThread* innerFlowThread = toLayoutBlockFlow(box).multiColumnFlowThread();
+    if (!innerFlowThread || innerFlowThread->isLayoutPagedFlowThread())
+        return;
+    LayoutUnit offsetInInnerFlowThread = flowThreadOffset() - innerFlowThread->blockOffsetInEnclosingFragmentationContext();
+    LayoutUnit innerUnbreakableHeight = innerFlowThread->tallestUnbreakableLogicalHeight(offsetInInnerFlowThread);
+    m_tallestUnbreakableLogicalHeight = std::max(m_tallestUnbreakableLogicalHeight, innerUnbreakableHeight);
 }
 
 void InitialColumnHeightFinder::examineBoxBeforeLeaving(const LayoutBox& box)
 {
 }
 
 static inline LayoutUnit columnLogicalHeightRequirementForLine(const ComputedStyle& style, const RootInlineBox& lastLine)
 {
     // We may require a certain minimum number of lines per page in order to satisfy
     // orphans and widows, and that may affect the minimum page height.
@@ skipping 136 matching lines @@
             // 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)
+    if (!flowThread || flowThread->isLayoutPagedFlowThread())
         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)
@@ skipping 22 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