Isolate strictly unbreakable multicol containers that are nested.

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 594 matching lines @@
     // fragmentation contexts. They are to be treated as unbreakable content.
     return nullptr;
   }
   if (multiColumnBlockFlow()->isInsideFlowThread())
     return toLayoutMultiColumnFlowThread(
         locateFlowThreadContainingBlockOf(*multiColumnBlockFlow()));
   return nullptr;
 }
 
 FragmentationContext*
-LayoutMultiColumnFlowThread::enclosingFragmentationContext() const {
-  if (LayoutMultiColumnFlowThread* enclosingFlowThread =
-          this->enclosingFlowThread())
+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())
     return enclosingFlowThread;
   return view()->fragmentationContext();
 }
 
 void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(
     LayoutUnit offsetInFlowThread,
     PageBoundaryRule pageBoundaryRule) {
   LayoutMultiColumnSet* columnSet =
       columnSetAtBlockOffset(offsetInFlowThread, pageBoundaryRule);
   if (!columnSet->newFragmentainerGroupsAllowed())
@@ skipping 742 matching lines @@
     const {
   return MultiColumnLayoutState(m_lastSetWorkedOn);
 }
 
 void LayoutMultiColumnFlowThread::restoreMultiColumnLayoutState(
     const MultiColumnLayoutState& state) {
   m_lastSetWorkedOn = state.columnSet();
 }
 
 }  // namespace blink