Controls if scroll commands move point to keep its screen position unchanged.
A value of nil means point does not keep its screen position except
at the scroll margin or window boundary respectively.
A value of t means point keeps its screen position if the scroll
command moved it vertically out of the window, e.g. when scrolling
by full screens.
Any other value means point always keeps its screen position.
Scroll commands should have the `scroll-command' property
on their symbols to be controlled by this variable.