"Throw" the page according to a mouse drag.
A "throw" is scrolling the page at a speed relative to the distance
from the original mouse click to the current mouse location. Try it;
you'll like it. It's easier to observe than to explain.
If the mouse is clicked and released in the same place of time we
assume that the user didn't want to scroll but wanted to whatever
mouse-2 used to do, so we pass it through.
Throw scrolling was inspired (but is not identical to) the "hand"
option in MacPaint, or the middle button in Tk text widgets.
If `mouse-throw-with-scroll-bar' is non-nil, then this command scrolls
in the opposite direction. (Different people have different ideas
about which direction is natural. Perhaps it has to do with which
hemisphere you're in.)
To test this function, evaluate:
(global-set-key [down-mouse-2] 'mouse-drag-throw)
(fn START-EVENT)