How far to scroll windows downward.
If you move point off the top, the window scrolls automatically.
This variable controls how far it scrolls. The value nil, the default,
means scroll to center point. A fraction means scroll to put point
that fraction of the window's height from the top of the window.
When the value is 0.0, point goes at the top line, which in the
simple case that you moved off with C-b means scrolling just one line.
1.0 means point goes at the bottom, so that in that simple case, the
window scrolls by a full window height. Meaningful values are between 0.0 and 1.0, inclusive.