Emacs performs automatic scrolling when point moves out of the visible portion of the text. Normally, automatic scrolling centers point vertically in the window, but there are several ways to alter this behavior.
If you set scroll-conservatively to a small number n,
then moving point just a little off the screen (no more than n
lines) causes Emacs to scroll just enough to bring point back on
screen; if doing so fails to make point visible, Emacs scrolls just
far enough to center point in the window. If you set
scroll-conservatively to a large number (larger than 100),
automatic scrolling never centers point, no matter how far point
moves; Emacs always scrolls text just enough to bring point into view,
either at the top or bottom of the window depending on the scroll
direction. By default, scroll-conservatively is 0, which
means to always center point in the window.
This said, in minibuffer windows, scrolling is always conservative by
default because scroll-minibuffer-conservatively is non-nil,
which takes precedence over scroll-conservatively.
Another way to control automatic scrolling is to customize the
variable scroll-step. Its value determines the number of lines
by which to automatically scroll, when point moves off the screen. If
scrolling by that number of lines fails to bring point back into view,
point is centered instead. The default value is zero, which (by
default) causes point to always be centered after scrolling.
A third way to control automatic scrolling is to customize the
variables scroll-up-aggressively and
scroll-down-aggressively, which directly specify the vertical
position of point after scrolling. The value of
scroll-up-aggressively should be either nil (the
default), or a floating point number f between 0 and 1. The
latter means that when point goes below the bottom window edge (i.e.,
scrolling forward), Emacs scrolls the window so that point is f
parts of the window height from the bottom window edge. Thus, larger
f means more aggressive scrolling: more new text is brought into
view. The default value, nil, is equivalent to 0.5.
Likewise, scroll-down-aggressively is used when point goes
above the top window edge (i.e., scrolling backward). The value
specifies how far point should be from the top margin of the window
after scrolling. Thus, as with scroll-up-aggressively, a
larger value is more aggressive.
Note that the variables scroll-conservatively,
scroll-step, and scroll-up-aggressively /
scroll-down-aggressively control automatic scrolling in
contradictory ways. Therefore, you should pick no more than one of
these methods to customize automatic scrolling. In case you customize
multiple variables, the order of priority is:
scroll-conservatively, then scroll-step, and finally
scroll-up-aggressively / scroll-down-aggressively.
The variable scroll-margin restricts how close point can come
to the top or bottom of a window (even if aggressive scrolling
specifies a fraction f that is larger than the window portion
between the top and the bottom margins). 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. The effective margin size is limited to a
quarter of the window height by default, but this limit can be
increased up to half (or decreased down to zero) by customizing
maximum-scroll-margin.