Strictly unbreakable objects need to prevent interaction with the outside.

third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.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 242 matching lines @@
   bool columnHeightsChanged() const { return m_columnHeightsChanged; }
   void setColumnHeightsChanged() { m_columnHeightsChanged = true; }
 
   void columnRuleStyleDidChange();
 
   // Remove the spanner placeholder and return true if the specified object is
   // no longer a valid spanner.
   bool removeSpannerPlaceholderIfNoLongerValid(
       LayoutBox* spannerObjectInFlowThread);
 
-  // Search mode when looking for an enclosing fragmentation context.
-  enum AncestorSearchConstraint {
-    // No constraints. Sometimes we just want to find all enclosing
-    // fragmentation contexts, e.g. to calculate the accumulated visual
-    // translation.
-    AnyAncestor,
-
-    // Consider fragmentation contexts that are strictly unbreakable (seen from
-    // the outside) to be isolated from the rest, so that such fragmentation
-    // contexts don't participate in fragmentation of enclosing fragmentation
-    // contexts, apart from taking up space and otherwise being completely
-    // unbreakable. This is typically what we want to do during layout.
-    IsolateUnbreakableContainers,
-  };
-  LayoutMultiColumnFlowThread* enclosingFlowThread() const;
+  LayoutMultiColumnFlowThread* enclosingFlowThread(
+      AncestorSearchConstraint = IsolateUnbreakableContainers) const;
   FragmentationContext* enclosingFragmentationContext(
       AncestorSearchConstraint = IsolateUnbreakableContainers) const;
   LayoutUnit blockOffsetInEnclosingFragmentationContext() const {
     DCHECK(enclosingFragmentationContext(AnyAncestor));
     return m_blockOffsetInEnclosingFragmentationContext;
   }
 
   // If we've run out of columns in the last fragmentainer group (column row),
   // we have to insert another fragmentainer group in order to hold more
   // columns. This means that we're moving to the next outer column (in the
@@ skipping 96 matching lines @@
 // LayoutObject.
 DEFINE_TYPE_CASTS(LayoutMultiColumnFlowThread,
                   LayoutFlowThread,
                   object,
                   object->isLayoutMultiColumnFlowThread(),
                   object.isLayoutMultiColumnFlowThread());
 
 }  // namespace blink
 
 #endif  // LayoutMultiColumnFlowThread_h