From the Project View you can drag an existing interface A onto another existing interface B and this creates an inner interface A.B, but you can't drag B back out of A again to make B a top level interface. in fact, it's almost too easy to accidentally drag interfaces on top of other intefaces and let go; I thought I was over a directlry when I did it. Then I had to play "where'd my interface go?" Then when I found it, I couldn't drag it out. Mostly that's b/c I didn't know you could drag an interface over another interface as per above, but if I can drag it in, I should defineitely also be able to drag it out.