Well, thank you very much anyway.
I've come up with a workaround which is replacing the inactive window with an identical-looking picture. ill put it here in case it's useful to someone
first the player reads text on a message window and sees a picture of choices next to it.
when they press SPACE, the window changes size + position, and shows choices instead. a picture of an empty window takes its original place.
and when the player selects a choice in the now legit choice window, its size and position changes back and it shows relevant text. the picture of choices returns.
repeat ad nauseum