Strictly unbreakable objects need to prevent interaction with the outside.

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

 /*
  * 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 580 matching lines @@
       ->setNeedsLayoutAndPrefWidthsRecalc(
           LayoutInvalidationReason::ColumnsChanged);
 
   // Now generate a column set for this ex-spanner, if needed and none is there
   // for us already.
   flowThreadDescendantWasInserted(spannerObjectInFlowThread);
 
   return true;
 }
 
-LayoutMultiColumnFlowThread* LayoutMultiColumnFlowThread::enclosingFlowThread()
-    const {
+LayoutMultiColumnFlowThread* LayoutMultiColumnFlowThread::enclosingFlowThread(
+    AncestorSearchConstraint constraint) const {
   if (isLayoutPagedFlowThread()) {
     // Paged overflow containers should never be fragmented by enclosing
     // fragmentation contexts. They are to be treated as unbreakable content.
     return nullptr;
   }
-  if (multiColumnBlockFlow()->isInsideFlowThread())
-    return toLayoutMultiColumnFlowThread(
-        locateFlowThreadContainingBlockOf(*multiColumnBlockFlow()));
-  return nullptr;
+  if (!multiColumnBlockFlow()->isInsideFlowThread())
+    return nullptr;
+  return toLayoutMultiColumnFlowThread(
+      locateFlowThreadContainingBlockOf(*multiColumnBlockFlow(), constraint));
 }
 
 FragmentationContext*
 LayoutMultiColumnFlowThread::enclosingFragmentationContext(
     AncestorSearchConstraint constraint) const {
   // If this multicol container is strictly unbreakable (due to having
   // scrollbars, for instance), it's also strictly unbreakable in any outer
   // fragmentation context. As such, what kind of fragmentation that goes on
   // inside this multicol container is completely opaque to the ancestors.
   if (constraint == IsolateUnbreakableContainers &&
       multiColumnBlockFlow()->getPaginationBreakability() == ForbidBreaks)
     return nullptr;
-  if (auto* enclosingFlowThread = this->enclosingFlowThread())
+  if (auto* enclosingFlowThread = this->enclosingFlowThread(constraint))
     return enclosingFlowThread;
   return view()->fragmentationContext();
 }
 
 void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(
     LayoutUnit offsetInFlowThread,
     PageBoundaryRule pageBoundaryRule) {
   LayoutMultiColumnSet* columnSet =
       columnSetAtBlockOffset(offsetInFlowThread, pageBoundaryRule);
   if (!columnSet->newFragmentainerGroupsAllowed())
@@ skipping 741 matching lines @@
     const {
   return MultiColumnLayoutState(m_lastSetWorkedOn);
 }
 
 void LayoutMultiColumnFlowThread::restoreMultiColumnLayoutState(
     const MultiColumnLayoutState& state) {
   m_lastSetWorkedOn = state.columnSet();
 }
 
 }  // namespace blink