@node Display, Search, Registers, Top
@chapter Controlling the Display

  Since only part of a large buffer fits in the window, XEmacs tries to show
the part that is likely to be interesting.  The display control commands
allow you to specify which part of the text you want to see.

@table @kbd
@item C-l
Clear frame and redisplay, scrolling the selected window to center
point vertically within it (@code{recenter-top-bottom}).
@item C-v
@itemx pgdn
@itemx next
Scroll forward (a windowful or a specified number of lines) (@code{scroll-up}).
On most X keyboards, you can get this functionality using the key
labelled @samp{Page Down}, which generates either @kbd{next} or @kbd{pgdn}.
@item M-v
@itemx pgup
@itemx prior
Scroll backward (@code{scroll-down}).  On most X keyboards, you can get
this functionality using the key labelled @samp{Page Up}, which
generates either @kbd{prior} or @kbd{pgup}.
@item @var{arg} C-l
Scroll so point is on line @var{arg} (@code{recenter}).
@item C-x <
@itemx C-pgdn
@itemx C-next
Scroll text in current window to the left (@code{scroll-left}).
@item C-x >
@itemx C-pgup
@itemx C-prior
Scroll to the right (@code{scroll-right}).
@item C-x $
Make deeply indented lines invisible (@code{set-selective-display}).
@end table

@menu
* Scrolling::	           Moving text up and down in a window.
* Recentering::            A scroll command that centers the current line.
* Automatic Scrolling::    Redisplay scrolls text automatically when needed.
* Horizontal Scrolling::   Moving text left and right in a window.
* Selective Display::      Hiding lines with lots of indentation.
* Display Vars::           Information on variables for customizing display.
@end menu

@node Scrolling, Recentering, Display, Display
@section Scrolling

  If a buffer contains text that is too large to fit entirely within the
window that is displaying the buffer, XEmacs shows a contiguous section of
the text.  The section shown always contains point.

@cindex scrolling
  @dfn{Scrolling} means moving text up or down in the window so that
different parts of the text are visible.  Scrolling forward means that text
moves up, and new text appears at the bottom.  Scrolling backward moves
text down and new text appears at the top.

  Scrolling happens automatically if you move point past the bottom or top
of the window.  You can also explicitly request scrolling with the commands
in this section.

@ifinfo
@table @kbd
@item C-v
@itemx pgdn
@itemx next
Scroll forward (a windowful or a specified number of lines) (@code{scroll-up}).
@item M-v
@itemx pgup
@itemx prior
Scroll backward (@code{scroll-down}).
@end table
@end ifinfo

@kindex C-v
@kindex M-v
@kindex pgup
@kindex pgdn
@kindex next
@kindex prior
@findex scroll-up
@findex scroll-down
  The scrolling commands @kbd{C-v} and @kbd{M-v} let you move all the text
in the window up or down a few lines.  @kbd{C-v} (@code{scroll-up}) with an
argument shows you that many more lines at the bottom of the window, moving
the text and point up together as @kbd{C-l} might.  @kbd{C-v} with a
negative argument shows you more lines at the top of the window.
@kbd{Meta-v} (@code{scroll-down}) is like @kbd{C-v}, but moves in the
opposite direction.@refill

@vindex next-screen-context-lines
  To read the buffer a windowful at a time, use @kbd{C-v} with no
argument.  @kbd{C-v} takes the last two lines at the bottom of the
window and puts them at the top, followed by nearly a whole windowful of
lines not previously visible.  Point moves to the new top of the window
if it was in the text scrolled off the top.  @kbd{M-v} with no argument
moves backward with similar overlap.  The number of lines of overlap
across a @kbd{C-v} or @kbd{M-v} is controlled by the variable
@code{next-screen-context-lines}; by default, it is two.

@node Recentering, Automatic Scrolling, Scrolling, Display
@section Recentering

@table @kbd
@item C-l
Scroll the selected window so the current line is the center-most text
line; on subsequent consecutive invocations, make the current line the
top line, the bottom line, and so on in cyclic order.  Possibly
redisplay the screen too (@code{recenter-top-bottom}).

@item M-x recenter
Scroll the selected window so the current line is the center-most text
line.  Possibly redisplay the screen too.

@item C-M-l
Scroll heuristically to bring useful information onto the screen
(@code{reposition-window}).
@end table

@kindex C-l
@findex recenter-top-bottom
  The @kbd{C-l} (@code{recenter-top-bottom}) command @dfn{recenters}
the selected window, scrolling it so that the current screen line is
exactly in the center of the window, or as close to the center as
possible.

  Typing @kbd{C-l} twice in a row (@kbd{C-l C-l}) scrolls the window
so that point is on the topmost screen line.  Typing a third @kbd{C-l}
scrolls the window so that point is on the bottom-most screen line.
Each successive @kbd{C-l} cycles through these three positions.

@vindex recenter-positions
  You can change the cycling order by customizing the list variable
@code{recenter-positions}.  Each list element should be the symbol
@code{top}, @code{middle}, or @code{bottom}, or a number; an integer
means to move the line to the specified screen line, while a
floating-point number between 0.0 and 1.0 specifies a percentage of
the screen space from the top of the window.  The default,
@code{(middle top bottom)}, is the cycling order described above.
Furthermore, if you change the variable @code{scroll-margin} to a
non-zero value @var{n}, @kbd{C-l} always leaves at least @var{n}
screen lines between point and the top or bottom of the window
(@pxref{Automatic Scrolling}).

  You can also give @kbd{C-l} a prefix argument.  A plain prefix
argument, @kbd{C-u C-l}, simply recenters point.  A positive argument
@var{n} puts point @var{n} lines down from the top of the window.  An
argument of zero puts point on the topmost line.  A negative argument
@var{-n} puts point @var{n} lines from the bottom of the window.  When
given an argument, @kbd{C-l} does not clear the screen or cycle
through different screen positions.

@findex recenter
  The more primitive command @kbd{M-x recenter} behaves like
@code{recenter-top-bottom}, but does not cycle among screen positions.

@node Automatic Scrolling, Horizontal Scrolling, Recentering, Display
@section Automatic Scrolling

@vindex scroll-step
  Scrolling happens automatically if point has moved out of the visible
portion of the text when it is time to display.  Usually scrolling is
done  to put point vertically centered within the window.  However, if
the variable @code{scroll-step} has a non-zero value, an attempt is made to
scroll the buffer by that many lines; if that is enough to bring point back
into visibility, that is what happens.

  Scrolling happens automatically if point has moved out of the visible
portion of the text when it is time to display.  Usually scrolling is
done  to put point vertically centered within the window.  However, if
the variable @code{scroll-step} has a non-zero value, an attempt is made to
scroll the buffer by that many lines; if that is enough to bring point back
into visibility, that is what happens.

@vindex scroll-conservatively
  If you set @code{scroll-step} to a small value because you want to use 
arrow keys to scroll the screen without recentering, the redisplay
preemption will likely make XEmacs keep recentering the screen when
scrolling fast, regardless of @code{scroll-step}.  To prevent this, set
@code{scroll-conservatively} to a small value, which will have the
result of overriding the redisplay preemption.

@node Horizontal Scrolling, Selective Display, Automatic Scrolling, Display
@section Horizontal Scrolling

@ifinfo
@table @kbd
@item C-x <
Scroll text in current window to the left (@code{scroll-left}).
@item C-x >
Scroll to the right (@code{scroll-right}).
@end table
@end ifinfo

@kindex C-x <
@kindex C-x >
@findex scroll-left
@findex scroll-right
@cindex horizontal scrolling
  The text in a window can also be scrolled horizontally.  This means that
each line of text is shifted sideways in the window, and one or more
characters at the beginning of each line are not displayed at all.  When a
window has been scrolled horizontally in this way, text lines are truncated
rather than continued (@pxref{Continuation Lines}), with a @samp{$} appearing
in the first column when there is text truncated to the left, and in the
last column when there is text truncated to the right.

  The command @kbd{C-x <} (@code{scroll-left}) scrolls the selected
window to the left by @var{n} columns with argument @var{n}.  With no
argument, it scrolls by almost the full width of the window (two columns
less, to be precise).  @kbd{C-x >} (@code{scroll-right}) scrolls
similarly to the right.  The window cannot be scrolled any farther to
the right once it is displaying normally (with each line starting at the
window's left margin); attempting to do so has no effect.

@node Selective Display, Display Vars, Horizontal Scrolling, Display
@section Selective Display
@findex set-selective-display
@kindex C-x $

  XEmacs can hide lines indented more than a certain number
of columns (you specify how many columns).  This allows you  to get an
overview of a part of a program.

  To hide lines, type @kbd{C-x $} (@code{set-selective-display}) with a
numeric argument @var{n}.  (@xref{Arguments}, for information on giving
the argument.)  Lines with at least @var{n} columns of indentation
disappear from the screen.  The only indication of their presence are
three dots (@samp{@dots{}}), which appear at the end of each visible
line that is followed by one or more invisible ones.@refill

  The invisible lines are still present in the buffer, and most editing
commands see them as usual, so it is very easy to put point in the middle
of invisible text.  When this happens, the cursor appears at the end of the
previous line, after the three dots.  If point is at the end of the visible
line, before the newline that ends it, the cursor appears before the three
dots.

  The commands @kbd{C-n} and @kbd{C-p} move across the invisible lines
as if they were not there.

  To make everything visible again, type @kbd{C-x $} with no argument.

@node Display Vars,, Selective Display, Display
@section Variables Controlling Display

  This section contains information for customization only.  Beginning
users should skip it.

@vindex no-redraw-on-reenter
  When you reenter XEmacs after suspending, XEmacs normally clears the
screen and redraws the entire display.  On some terminals with more than
one page of memory, it is possible to arrange the termcap entry so that
the @samp{ti} and @samp{te} strings (output to the terminal when XEmacs
is entered and exited, respectively) switch between pages of memory so
as to use one page for XEmacs and another page for other output.  In that
case, you might want to set the variable @code{no-redraw-on-reenter} to
non-@code{nil} so that XEmacs will assume, when resumed, that the screen
page it is using still contains what XEmacs last wrote there.

@vindex echo-keystrokes
  The variable @code{echo-keystrokes} controls the echoing of multi-character
keys; its value is the number of seconds of pause required to cause echoing
to start, or zero, meaning don't echo at all.  @xref{Echo Area}.

@vindex ctl-arrow
  If the variable @code{ctl-arrow} is @code{nil}, control characters in the
buffer are displayed with octal escape sequences, all except newline and
tab.  If its value is @code{t}, then control characters will be printed 
with an up-arrow, for example @kbd{^A}.  

If its value is not @code{t} and not @code{nil}, then characters whose
code is greater than 160 (that is, the space character (32) with its
high bit set) will be assumed to be printable, and will be displayed
without alteration.  This is the default when running under X Windows,
since XEmacs assumes an ISO/8859-1 character set (also known as
``Latin1'').  The @code{ctl-arrow} variable may also be set to an
integer, in which case all characters whose codes are greater than or
equal to that value will be assumed to be printable.

Altering the value of @code{ctl-arrow} makes it local to the current
buffer; until that time, the default value is in effect.  @xref{Locals}.

@vindex tab-width
  Normally, a tab character in the buffer is displayed as whitespace which
extends to the next display tab stop position, and display tab stops come
at intervals equal to eight spaces.  The number of spaces per tab is
controlled by the variable @code{tab-width}, which is made local by
changing it, just like @code{ctl-arrow}.  Note that how the tab character
in the buffer is displayed has nothing to do with the definition of
@key{TAB} as a command.

@vindex selective-display-ellipses
  If you set the variable @code{selective-display-ellipses} to @code{nil},
the three dots at the end of a line that precedes invisible
lines do not appear.  There is no visible indication of the invisible lines.
This variable becomes local automatically when set.