i seem to remember that under nextstep the algorithm was something
like:

	while the mouse is below the window:
		at some repetition interval t, adjust the current y-coordinate
		so that the y-coord that was previously "under" the mouse
		is now at the bottom of the window.

that gives a fairly natural proportionality to the thing.  of course,
it's not that simple in acme/rio style text windows, where the
scrolling is proportional to the amount of data shown, not the
vertical space taken by the data.