Use the container() chain when locating the containing flow thread.

Source/core/layout/LayoutObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 658 matching lines @@
             return flowThread;
     }
 
     // Not in the middle of layout so have to find the thread the slow way.
     LayoutObject* curr = const_cast<LayoutObject*>(this);
     while (curr) {
         if (curr->isSVG() && !curr->isSVGRoot())
             return nullptr;
         if (curr->isLayoutFlowThread())
             return toLayoutFlowThread(curr);
-        LayoutObject* containingBlock = curr->containingBlock();
+        LayoutObject* container = curr->container();
         curr = curr->parent();
-        while (curr != containingBlock) {
+        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();
         }
     }
     return nullptr;
@@ skipping 2608 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         fprintf(stderr, "Cannot showLayoutTree. Root is (nil)\n");
     }
 }
 
 #endif