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

third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.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.
 
 #ifndef MultiColumnFragmentainerGroup_h
 #define MultiColumnFragmentainerGroup_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "wtf/Allocator.h"
 
@@ skipping 45 matching lines @@
   LayoutUnit logicalTopInFlowThread() const { return m_logicalTopInFlowThread; }
   void setLogicalTopInFlowThread(LayoutUnit logicalTopInFlowThread) {
     m_logicalTopInFlowThread = logicalTopInFlowThread;
   }
 
   // The bottom of our flow thread portion
   LayoutUnit logicalBottomInFlowThread() const {
     return m_logicalBottomInFlowThread;
   }
   void setLogicalBottomInFlowThread(LayoutUnit logicalBottomInFlowThread) {
-    ASSERT(logicalBottomInFlowThread >= m_logicalTopInFlowThread);
     m_logicalBottomInFlowThread = logicalBottomInFlowThread;
   }
 
-  // The height of our flow thread portion
+  // The height of the flow thread portion for the entire fragmentainer group.
   LayoutUnit logicalHeightInFlowThread() const {
-    return m_logicalBottomInFlowThread - m_logicalTopInFlowThread;
+    // Due to negative margins, logical bottom may actually end up above logical
+    // top, but we never want to return negative logical heights.
+    return (m_logicalBottomInFlowThread - m_logicalTopInFlowThread)
+        .clampNegativeToZero();
   }
+  // The height of the flow thread portion for the specified fragmentainer.
+  // The last fragmentainer may not be using all available space.
+  LayoutUnit logicalHeightInFlowThreadAt(unsigned columnIndex) const;
 
   void resetColumnHeight();
   bool recalculateColumnHeight(LayoutMultiColumnSet&);
 
   LayoutSize flowThreadTranslationAtOffset(LayoutUnit,
                                            LayoutBox::PageBoundaryRule,
                                            CoordinateSpaceConversion) const;
   LayoutUnit columnLogicalTopForOffset(LayoutUnit offsetInFlowThread) const;
 
   // If SnapToColumnPolicy is SnapToColumn, visualPointToFlowThreadPoint() won't
@@ skipping 115 matching lines @@
 
  private:
   LayoutMultiColumnSet& m_columnSet;
 
   Vector<MultiColumnFragmentainerGroup, 1> m_groups;
 };
 
 }  // namespace blink
 
 #endif  // MultiColumnFragmentainerGroup_h