If an object's containing block is in a flow thread, so is the object.

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

 /*
  * Copyright (C) 2007 Apple Inc.  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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 34 matching lines @@
 }
 
 LayoutState::LayoutState(LayoutBox& layoutObject,
                          LayoutUnit pageLogicalHeight,
                          bool containingBlockLogicalWidthChanged)
     : m_containingBlockLogicalWidthChanged(containingBlockLogicalWidthChanged),
       m_next(layoutObject.view()->layoutState()),
       m_layoutObject(layoutObject) {
   if (layoutObject.isLayoutFlowThread())
     m_flowThread = toLayoutFlowThread(&layoutObject);
-  else if (!layoutObject.isOutOfFlowPositioned())
+  else
     m_flowThread = m_next->flowThread();
-  else
-    m_flowThread = nullptr;
   m_paginationStateChanged = m_next->m_paginationStateChanged;
   layoutObject.view()->pushLayoutState(*this);
   m_heightOffsetForTableHeaders = m_next->heightOffsetForTableHeaders();
 
   if (pageLogicalHeight || layoutObject.isLayoutFlowThread()) {
     // Entering a new pagination context.
     m_pageLogicalHeight = pageLogicalHeight;
     m_paginationOffset = LayoutSize();
     m_isPaginated = true;
     return;
@@ skipping 65 matching lines @@
 
 LayoutUnit LayoutState::pageLogicalOffset(
     const LayoutBox& child,
     const LayoutUnit& childLogicalOffset) const {
   if (child.isHorizontalWritingMode())
     return m_paginationOffset.height() + childLogicalOffset;
   return m_paginationOffset.width() + childLogicalOffset;
 }
 
 }  // namespace blink