useCameraFollow with rooms cuts the level into rooms the size of the screen (a list of rectangles also works, for rooms of any size). The camera stays in the room the hero is in, and when the hero walks out of it, it scrolls across to the next in half a second (scroll).
The hero waits while the screen scrolls, reading follow.scrolling, and the counter in the corner changes as each room is entered (onRoom).
← → walk, Z jump.