[XUL Tree] missing twisty after transforming a leaf into a container

Hi,

We're using a RDF tree for displaying objects hierarchy.
It loads a main RDF on user query, then it adds other RDFs when user 
open nodes which don't have children loaded yet.

This basically works (thanks to answer in the thread 
<[email protected]> 02/09/2012)

I'm facing something I didn't notice at this time: the user selects a 
leaf object C (rdf:li) and wants to create a child.

A
|->B
    |-C

AFter the creation (server-side), we then reload this part of the graph 
but the C object will still look like a leaf (no twisty).
If I dbl-click on it, it will open and reveal the "just created object".

I tried to add an Unassert call before adding the RDF, with no luck.

It will only appear if I completely reload the tree (remove all 
datasources, and add one new).

I don't know what could cause this behaviour.


Thanks for reading and helping :)