*** DO NOT LAND *** Attempt to understand Regions complexity

Source/core/rendering/RenderGeometryMap.cpp

 /*
  * Copyright (C) 2012 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 183 matching lines @@
     ASSERT(m_mapping.isEmpty() || isTopmostRenderView(m_mapping[0].m_renderer));
 }
 
 static bool canMapBetweenRenderers(const RenderObject* renderer, const RenderObject* ancestor)
 {
     for (const RenderObject* current = renderer; ; current = current->parent()) {
         const RenderStyle* style = current->style();
         if (style->position() == FixedPosition || style->isFlippedBlocksWritingMode())
             return false;
 
-        if (current->hasColumns() || current->hasTransform() || current->isRenderFlowThread() || current->isSVGRoot())
+        if (current->hasColumns() || current->hasTransform() || current->isSVGRoot())
             return false;
 
         if (current == ancestor)
             break;
     }
 
     return true;
 }
 
 void RenderGeometryMap::pushMappingsToAncestor(const RenderLayer* layer, const RenderLayer* ancestorLayer)
@@ skipping 122 matching lines @@
     // If we're not working with multiple RenderViews, then any view is considered
     // "topmost" (to preserve original behavior).
     if (!(m_mapCoordinatesFlags & TraverseDocumentBoundaries))
         return true;
 
     return renderer->frame()->isMainFrame();
 }
 #endif
 
 } // namespace WebCore