Finding track index for boxed tracks

Dont know whether Im just tired…but I cannot find the right way to simply extract the global track from the box offset?

ie self.song().tracks[self.track_num(0)].???idx???

Where would I find this?
What Im trying to do is simply test whether the selected track is in a box or not?
I need to know the bounds of the first and last track in the box.

I want to move a box so that the edges of the box will push the selected track/index along ie If a box moves along, it should grab and keep the selected track within the box

eg selected track is currently number 2 in the box.
I move the box 3 tracks to the right
on the 3rd move, the track is also pushed along with the box so it doesnt fall outside the bounds of the box

Hope that makes sense