Yep, I knew that from the other thread.
So, my question is, from the list returned, how can I identify the active screen?
I don’t know that you can — you probably need to call mainScreen separately. Perhaps:
set theRects to current application's NSScreen's screens()'s valueForKey:"frame"
set mainRect to current application's NSScreen's mainScreen()'s frame()
set rectsLessMain to theRects's mutableCopy()
rectsLessMain's removeObject:(current application's NSValue's valueWithRect:mainRect)