Strictly unbreakable objects need to prevent interaction with the outside.

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

 /*
  * 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 53 matching lines @@
 // LayoutMultiColumnSet will actually be a viewPort of the LayoutFlowThread.
 class CORE_EXPORT LayoutFlowThread : public LayoutBlockFlow {
  public:
   LayoutFlowThread();
   ~LayoutFlowThread() override {}
 
   bool isLayoutFlowThread() const final { return true; }
   virtual bool isLayoutMultiColumnFlowThread() const { return false; }
   virtual bool isLayoutPagedFlowThread() const { return false; }
 
+  // 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,
+  };
+
   static LayoutFlowThread* locateFlowThreadContainingBlockOf(
-      const LayoutObject&);
+      const LayoutObject&,
+      AncestorSearchConstraint = IsolateUnbreakableContainers);
 
   void layout() override;
 
   // Always create a Layer for the LayoutFlowThread so that we
   // can easily avoid drawing the children directly.
   PaintLayerType layerTypeRequired() const final { return NormalPaintLayer; }
 
   virtual void flowThreadDescendantWasInserted(LayoutObject*) {}
   virtual void flowThreadDescendantWillBeRemoved(LayoutObject*) {}
   virtual void flowThreadDescendantStyleWillChange(
@@ skipping 118 matching lines @@
 struct ValueToString<LayoutMultiColumnSet*> {
   static String toString(const LayoutMultiColumnSet* value) {
     return String::format("%p", value);
   }
 };
 #endif
 
 }  // namespace blink
 
 #endif  // LayoutFlowThread_h