Emacs performs automatic scrolling when point moves out of the visible portion of the text.
Normally, this centers point vertically within the window. However,
if you set scroll-conservatively to a small number n,
then if you move point just a little off the screen (less than n
lines), Emacs scrolls the text just far enough to bring point back on
screen. By default, scroll-conservatively is 0.
When the window does scroll by a longer distance, you can control
how aggressively it scrolls by setting the variables
scroll-up-aggressively and scroll-down-aggressively.
The value of scroll-up-aggressively should be either
nil, or a fraction f between 0 and 1. A fraction
specifies where on the screen to put point when scrolling upward: when
a window scrolls up because point is above the window start, the new
start position is chosen to put point f parts of the window
height from the top. Thus, larger f means more aggressive
scrolling. The default value, nil, is equivalent to 0.5.
Likewise, scroll-down-aggressively is used for scrolling
down. The value specifies how far point should be placed from the
bottom of the window; thus, as with scroll-up-aggressively, a
larger value is more aggressive.
The variable scroll-margin restricts how close point can come
to the top or bottom of a window. Its value is a number of screen
lines; if point comes within that many lines of the top or bottom of
the window, Emacs performs automatic scrolling. By default,
scroll-margin is 0.