Issue opening and closing folder nodes with only a folder inside

[grouss]

It seems I have a small bug or misunderstanding of the specs.
Clicking on a folder node which only contains another folder node, I see a "free node" that is still displayed.
Before the clik

after the click

Is it a bug or is it part of the spec that would not allow folder node without leaf ?
It seems ok with folders containing two other folder nodes

[grouss]

by the way I was able to reproduce it with the demo available here http://www.redotheweb.com/CodeFlower/