Hi Pilou, 
 
> When you move A over b, or Draw A from the middle of B 
> Automatically segmented and "welded! A et B don't exist anymore ! 
> so when you move the blue part, that make a sort of trapeze! 
 
Yup, but try moving A over B after having made A and B each into a separate group and you should see different behavior. 
 
   - Michael 
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