A container of out-of-flow positioned descendants should be an offsetParent.

third_party/WebKit/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 2938 matching lines @@
         // TODO(kochi): If |base| or |node| is nested deep in shadow roots, this loop may
         // get expensive, as isUnclosedNodeOf() can take up to O(N+M) time (N and M are depths).
         if (base && (node->isClosedShadowHiddenFrom(*base) || (node->isInShadowTree() && node->containingShadowRoot()->type() == ShadowRootType::UserAgent))) {
             // If 'position: fixed' node is found while traversing up, terminate the loop and
             // return null.
             if (ancestor->isFixedPositioned())
                 return nullptr;
             continue;
         }
 
-        if (ancestor->isPositioned())
+        if (ancestor->canContainAbsolutePositionObjects())
             break;
 
         if (isHTMLBodyElement(*node))
             break;
 
         if (!isPositioned() && (isHTMLTableElement(*node) || isHTMLTableCellElement(*node)))
             break;
 
         // Webkit specific extension where offsetParent stops at zoom level changes.
         if (effectiveZoom != ancestor->style()->effectiveZoom())
@@ skipping 299 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         WTFLogAlways("%s", "Cannot showLayoutTree. Root is (nil)");
     }
 }
 
 #endif