Strictly unbreakable objects need to prevent interaction with the outside.

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

Index: third_party/WebKit/Source/core/layout/LayoutFlowThread.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutFlowThread.h b/third_party/WebKit/Source/core/layout/LayoutFlowThread.h
index f0087d0d797398b942e8cffa7955b9ab0c269706..3654b455d19c0602101bdecf20883e24c5c8fd07 100644
--- a/third_party/WebKit/Source/core/layout/LayoutFlowThread.h
+++ b/third_party/WebKit/Source/core/layout/LayoutFlowThread.h
@@ -71,8 +71,24 @@ class CORE_EXPORT LayoutFlowThread : public LayoutBlockFlow {
   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;