If "AutoScroll" is selected, then all MB3 clicks start
the autoscroll mode if the mouse pointer is over a scrollable area.
While in auto-scroll mode, moving the pointer scrolls the area. The farthest
the pointer is from its initial position, the fastest the area scrolls.
Clicking any mouse button exits the autoscroll mode.
Note: This does not interfere with the "MB3 drag scrolling" settings below
that determine what happens when you drag the mouse while mouse button 3
is down. You can have AutoScroll enabled and drag-scrolling disabled, and vice
versa.