I was in 12D on a flight last week on a 320 and the location of the cabin divider meant that I couldn't see the screen at all - I'd have had to be sitting in 12E's lap to see it. Not a big deal but I guess there's often issues where screens can't be seen / don't drop / have screen shake (lots of them seem to have that). I guess it's seen as being good enough if it's clearly visible for for 90% + of passengers. Don't know what the absolute rules are..