Perhaps it's a recent change, but they DO sell popcorn for the movies now. They have a small roller cart they set up in front of the door, and they have popcorn and a couple other small items (don't remember as we didn't buy those). They write up the charge manually as "room service", so I think we paid slightly more for it there than we did in front of the Walt Disney Theatre.
Just as a point of reference, this was the case on the 09/28 "mystery cruise". I can't tell you how long they had been doing it prior to that cruise, nor if they are still doing it for the three or four cruises since then...