Strictly unbreakable objects need to prevent interaction with the outside.

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

 /*
  * Copyright (C) 2011 Adobe Systems Incorporated. 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.
@@ skipping 22 matching lines @@
 #include "core/layout/LayoutMultiColumnSet.h"
 
 namespace blink {
 
 LayoutFlowThread::LayoutFlowThread()
     : LayoutBlockFlow(nullptr),
       m_columnSetsInvalidated(false),
       m_pageLogicalSizeChanged(false) {}
 
 LayoutFlowThread* LayoutFlowThread::locateFlowThreadContainingBlockOf(
-    const LayoutObject& descendant) {
+    const LayoutObject& descendant,
+    AncestorSearchConstraint constraint) {
   ASSERT(descendant.isInsideFlowThread());
   LayoutObject* curr = const_cast<LayoutObject*>(&descendant);
   while (curr) {
     if (curr->isSVGChild())
       return nullptr;
     if (curr->isLayoutFlowThread())
       return toLayoutFlowThread(curr);
     LayoutObject* container = curr->container();
+    // If we're inside something strictly unbreakable (due to having scrollbars
+    // or being writing mode roots, for instance), it's also strictly
+    // unbreakable in any outer fragmentation context. As such, what goes on
+    // inside any fragmentation context on the inside of this is completely
+    // opaque to ancestor fragmentation contexts.
+    if (constraint == IsolateUnbreakableContainers && container &&
+        container->isBox() &&
+        toLayoutBox(container)->getPaginationBreakability() == ForbidBreaks)
+      return nullptr;
     curr = curr->parent();
     while (curr != container) {
       if (curr->isLayoutFlowThread()) {
         // The nearest ancestor flow thread isn't in our containing block chain.
         // Then we aren't really part of any flow thread, and we should stop
         // looking. This happens when there are out-of-flow objects or column
         // spanners.
         return nullptr;
       }
       curr = curr->parent();
@@ skipping 170 matching lines @@
 
 void LayoutFlowThread::MultiColumnSetSearchAdapter::collectIfNeeded(
     const MultiColumnSetInterval& interval) {
   if (m_result)
     return;
   if (interval.low() <= m_offset && interval.high() > m_offset)
     m_result = interval.data();
 }
 
 }  // namespace blink