Ok so I took a look at Fetish Nightfall. Not the code, but just loaded up and used the UI in game to see how it behaved. Slightly similiar to what i'm doing and I see how you did the whole docking window thing from the user perspective. Though what I noticed was that the collapse function rolls down the inventory similiar to what i'm doing but there was no dummy window that I could see, from the perspective of the user, that dictated where that ended up.
So I guess i'll take alook at the code and see what you did