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 131 matching lines @@
                                         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 {
-    return logicalBottomInFlowThread() - logicalTopInFlowThread();
+    // Due to negative margins, logical bottom may actually end up above logical
+    // top, but we never want to return negative logical heights.
+    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