This is probably too complicated to implement, but maybe I'm wrong.
Ah, I see. Looks not bad either, but I prefer the other variant, since it's going to be too much white, when you have more than just one notenook open (on level 1). But this is a matter of taste, and as you know, there's no accounting for taste. So I guess we'll have to wait and see how it's going to be implemented.
Maybe it's even possible to make this configurable. But I guess this would make it only more complex.