Allow flow thread portion logical bottom to be above its logical top.

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

 /*
  * 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 130 matching lines @@
   bool hasFragmentainerGroupForColumnAt(LayoutUnit bottomOffsetInFlowThread,
                                         PageBoundaryRule) const;
 
   MultiColumnFragmentainerGroup& appendNewFragmentainerGroup();
 
   // Logical top relative to the content edge of the multicol container.
   LayoutUnit logicalTopFromMulticolContentEdge() const;
 
   LayoutUnit logicalTopInFlowThread() const;
   LayoutUnit logicalBottomInFlowThread() const;
   LayoutUnit logicalHeightInFlowThread() const {

[eae, 2017/02/23 22:20:20]

Would you mind adding a comment here explaining how this could be negative (i.e.
if the top is above the bottom). For most casual readers that would seem to be
impossible thus making this check seem unnecessary. 

[mstensho (USE GERRIT), 2017/02/27 11:57:05]

Good idea. Done.

-    return logicalBottomInFlowThread() - logicalTopInFlowThread();
+    return (logicalBottomInFlowThread() - logicalTopInFlowThread())
+        .clampNegativeToZero();
   }
 
   // Return the amount of flow thread contents that the specified fragmentainer
   // group can hold without overflowing.
   LayoutUnit fragmentainerGroupCapacity(
       const MultiColumnFragmentainerGroup& group) const {
     return group.logicalHeight() * usedColumnCount();
   }
 
   LayoutRect flowThreadPortionRect() const;
@@ skipping 110 matching lines @@
   LayoutUnit m_oldLogicalTop;
 
   bool m_initialHeightCalculated;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSet, isLayoutMultiColumnSet());
 
 }  // namespace blink
 
 #endif  // LayoutMultiColumnSet_h