Even empty block intervals should be associated with a fragmentainer.

third_party/WebKit/Source/core/layout/FragmentainerIterator.cpp

 // Copyright 2016 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/FragmentainerIterator.h"
 
 #include "core/layout/LayoutMultiColumnSet.h"
 
 namespace blink {
 
@@ skipping 11 matching lines @@
   m_flowThread.flipForWritingMode(boundsInFlowThread);
 
   if (m_flowThread.isHorizontalWritingMode()) {
     m_logicalTopInFlowThread = boundsInFlowThread.y();
     m_logicalBottomInFlowThread = boundsInFlowThread.maxY();
   } else {
     m_logicalTopInFlowThread = boundsInFlowThread.x();
     m_logicalBottomInFlowThread = boundsInFlowThread.maxX();
   }
 
   // Jump to the first interesting column set.

[szager1, 2016/12/21 23:47:11]

This comment seems obsolete.

[mstensho (USE GERRIT), 2016/12/22 08:10:26]

Not really. We're still picking a column set based on the flow thread range
start.

   m_currentColumnSet = flowThread.columnSetAtBlockOffset(
       m_logicalTopInFlowThread, LayoutBox::AssociateWithLatterPage);
-  if (!m_currentColumnSet ||
-      m_currentColumnSet->logicalTopInFlowThread() >=
-          m_logicalBottomInFlowThread) {
+  if (!m_currentColumnSet) {
     setAtEnd();
     return;
   }
   // Then find the first interesting fragmentainer group.
   m_currentFragmentainerGroupIndex =
       m_currentColumnSet->fragmentainerGroupIndexAtFlowThreadOffset(
           m_logicalTopInFlowThread, LayoutBox::AssociateWithLatterPage);
 
   // Now find the first and last fragmentainer we're interested in. We'll also
   // clip against the clip rect here. In case the clip rect doesn't intersect
@@ skipping 108 matching lines @@
     if (m_currentFragmentainerIndex < firstFragmentainerInClipRect)
       m_currentFragmentainerIndex = firstFragmentainerInClipRect;
     if (m_endFragmentainerIndex > lastFragmentainerInClipRect)
       m_endFragmentainerIndex = lastFragmentainerInClipRect;
   }
   DCHECK(m_endFragmentainerIndex >= m_currentFragmentainerIndex);
   return true;
 }
 
 }  // namespace blink