Calling doc.replaceChild(new_child, old_child) with new_child and old_child being similar nodes in two different documents results in new_child to be removed from its document instead of old_child being removed from doc.

Example:
        new_child = get_element_x(doc_a)
        old_child = get_element_x(doc_b)
        parent = get_element_y(doc_b)
        parent.replaceChild(new_child, old_child)

=> new_child will be removed from doc_a. Instead old_child should be removed from doc_b.

Who is best placed to look at this as nobody is listed on the experts index?

Since "new_child" is inserted at least into a new place, it needs to be removed from its old place as well, so that part seems correct. The problem description does not make it clear whether or not "old_child" is handled correctly, but looking at the code (and lacking a complete example that shows an error), I cannot see anything wrong there.

I'll close this ticket since it has been idle for years and it is unclear to me from the description whether it is an actual misbehaviour or a misunderstanding of correct behaviour.