Only allow forced fragmentainer breaks at class A break points.

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

 // 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 "core/layout/MultiColumnFragmentainerGroup.h"
 
 namespace blink {
 
 // A column balancer traverses the portion of the subtree of a flow thread that belongs to a given
 // fragmentainer group, in order to collect certain data to be used for column balancing. This is an
 // abstract class that just walks the subtree and leaves it to subclasses to actualy collect data.
 class ColumnBalancer {
 protected:
     ColumnBalancer(const MultiColumnFragmentainerGroup&);
 
     const MultiColumnFragmentainerGroup& group() const { return m_group; }
 
     // Flow thread offset for the layout object that we're currently examining.
     LayoutUnit flowThreadOffset() const { return m_flowThreadOffset; }
 
+    EBreak previousBreakAfterValue() const { return m_previousBreakAfterValue; }
+
     // Return true if the specified offset is at the top of a column, as long as it's not the first
     // column in the fragmentainer group.
     bool isFirstAfterBreak(LayoutUnit flowThreadOffset) const
     {
         if (flowThreadOffset != m_group.columnLogicalTopForOffset(flowThreadOffset))
             return false; // Not at the top of a column.
         // The first column in the fragmentainer group is either not after any break at all, or
         // after a break that belongs to the previous fragmentainer group.
         return flowThreadOffset > m_group.logicalTopInFlowThread();
     }
@@ skipping 25 matching lines @@
     // Examine and collect column balancing data for everything in the fragmentainer group. Will
     // trigger calls to examineBoxAfterEntering(), examineBoxBeforeLeaving() and examineLine() for
     // interesting boxes and lines.
     void traverse();
 
 private:
     void traverseSubtree(const LayoutBox&);
 
     const MultiColumnFragmentainerGroup& m_group;
     LayoutUnit m_flowThreadOffset;
+
+    // The break-after value from the previous in-flow block-level object to be joined with the
+    // break-before value of the next in-flow block-level object.
+    EBreak m_previousBreakAfterValue;
 };
 
 // After an initial layout pass, we know the height of the contents of a flow thread. Based on
 // this, we can estimate an initial minimal column height. This class will collect the necessary
 // information from the layout objects to make this estimate. This estimate may be used to perform
 // another layout iteration. If we after such a layout iteration cannot fit the contents with the
 // given column height without creating overflowing columns, we will have to stretch the columns by
 // some amount and lay out again. We may need to do this several times (but typically not more
 // times than the number of columns that we have). The amount to stretch is provided by the sister
 // of this class, named MinimumSpaceShortageFinder.
@@ skipping 113 matching lines @@
     LayoutUnit m_minimumSpaceShortage;
 
     // Set when breaking before a block, and we're looking for the first unbreakable descendant, in
     // order to report correct space shortage for that one.
     LayoutUnit m_pendingStrut;
 
     unsigned m_forcedBreaksCount;
 };
 
 } // namespace blink