Once a file has been loaded, the screen is divided up into 3 equal
horizontal areas. The top and bottom areas are sub divided into 5 zones,
each with their own scrolling speed. The closer you get to the screen
boundary, the faster the scoll. Needless to say, the top area moves towards
the top of the file and the bottom area moves towards the end of the file.
The middle area is neutral and all scrolling will stop.