BlitzUI is nice for very simple interfaces but its very unstable and gets confused easily.
I find BlitzUI is very stable and never gets 'confused'. Not sure what you mean by confused ??
As for 'simple interfaces' - well that's purely down to the skill of the programmer !
BlitzUI is a 2d GUI - the interface is drawn in 2d using 2d commands over 3d whereas F-UI is a pure 3d GUI - there are no 2d drawing commands and so is much faster.
p.s Your app doesn't need to know exactly which window its working with it just needs to know what gadget or menu option has been triggered via the events. It would also be fairly easy to keep track of what windows are open by having a boolean variable that is set to true when a window is opened and then set to false when it is closed but I can't think of many scenarios where this would be necessary.
The important thing your app needs to know is what Gadget has been clicked rather than the window itself.
p.p.s.
I see you repackaged the same question with a different title ! I wouldn't have answered your previous post due to its demanding and patronising "Oh so nobody knows !" comments. I think that you probably understand now that that type of post generally gets NO response at all and only serves to annoy people who would have probably helped if you'd had a bit of patience. This is not a help desk but a forum - you cannot demand an answer.