(Font Lock): Mention jit-lock and its customization group.

[gnu-emacs] / man / display.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997, 2000, 2001
@c   Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Display, Search, Registers, Top
@chapter Controlling the Display

  Since only part of a large buffer fits in the window, Emacs tries to
show a part that is likely to be interesting.  Display-control commands
allow you to specify which part of the text you want to see, and how to
display it.

@menu
* Faces::                  How to change the display style using faces.
* Font Lock::              Minor mode for syntactic highlighting using faces.
* Highlight Changes::      Using colors to show where you changed the buffer.
* Highlight Interactively:: Tell Emacs what text to highlight.
* Trailing Whitespace::    Showing possibly-spurious trailing whitespace.
* Scrolling::              Moving text up and down in a window.
* Horizontal Scrolling::   Moving text left and right in a window.
* Follow Mode::            Follow mode lets two windows scroll as one.
* Selective Display::      Hiding lines with lots of indentation.
* Optional Mode Line::     Optional mode line display features.
* Text Display::           How text characters are normally displayed.
* Display Custom::         Information on variables for customizing display.
* Cursor Display::         Features for displaying the cursor.
@end menu

@node Faces
@section Using Multiple Typefaces
@cindex faces

  When using Emacs with a window system, you can set up multiple
styles of displaying characters.  Each style is called a @dfn{face}.
Each face can specify various attributes, such as the height, weight
and slant of the characters, the foreground and background color, and
underlining.  But it does not have to specify all of them.

  Features which rely on text in multiple faces (such as Font Lock mode)
will also work on non-windowed terminals that can display more than one
face, whether by colors or underlining and emboldening.  This includes
the console on GNU/Linux, an @code{xterm} which supports colors, the
MS-DOS display (@pxref{MS-DOS}), and the MS-Windows version invoked with
the @option{-nw} option.  Emacs determines automatically whether the
terminal has this capability.

  You control the appearance of a part of the text in the buffer by
specifying the face or faces to use for it.  The style of display used
for any given character is determined by combining the attributes of
all the applicable faces specified for that character.  Any attribute
that isn't specified by these faces is taken from the @code{default} face,
whose attributes reflect the default settings of the frame itself.

  Enriched mode, the mode for editing formatted text, includes several
commands and menus for specifying faces for text in the buffer.
@xref{Format Faces}, for how to specify the font for text in the
buffer.  @xref{Format Colors}, for how to specify the foreground and
background color.

@cindex face colors, setting
@findex set-face-foreground
@findex set-face-background
  To alter the appearance of a face, use the customization buffer.
@xref{Face Customization}.  You can also use X resources to specify
attributes of particular faces (@pxref{Resources}).  Alternatively,
you can change the foreground and background colors of a specific face
with @kbd{M-x set-face-foreground} and @kbd{M-x set-face-background}.
These commands prompt in the minibuffer for a face name and a color
name, with completion, and then set that face to use the specified
color.  Changing the colors of the @code{default} face also changes
the foreground and background colors on all frames, both existing and
those to be created in the future.  (You can also set foreground and
background colors for the current frame only; see @ref{Frame
Parameters}.)

  Emacs 21 can correctly display variable-width fonts, but Emacs
commands that calculate width and indentation do not know how to
calculate variable widths.  This can sometimes lead to incorrect
results when you use variable-width fonts.  In particular, indentation
commands can give inconsistent results, so we recommend you avoid
variable-width fonts for editing program source code.  Filling will
sometimes make lines too long or too short.  We plan to address these
issues in future Emacs versions.

@findex list-faces-display
  To see what faces are currently defined, and what they look like, type
@kbd{M-x list-faces-display}.  It's possible for a given face to look
different in different frames; this command shows the appearance in the
frame in which you type it.  Here's a list of the standard defined
faces:

@table @code
@item default
This face is used for ordinary text that doesn't specify any other face.
@item mode-line
This face is used for mode lines.  By default, it's drawn with shadows
for a ``raised'' effect on window systems, and drawn as the inverse of
the default face on non-windowed terminals.  @xref{Display Custom}.
@item header-line
Similar to @code{mode-line} for a window's header line.  Most modes
don't use the header line, but the Info mode does.
@item highlight
This face is used for highlighting portions of text, in various modes.
For example, mouse-sensitive text is highlighted using this face.
@item isearch
This face is used for highlighting Isearch matches.
@item isearch-lazy-highlight-face
This face is used for lazy highlighting of Isearch matches other than
the current one.
@item region
This face is used for displaying a selected region (when Transient Mark
mode is enabled---see below).
@item secondary-selection
This face is used for displaying a secondary X selection (@pxref{Secondary
Selection}).
@item bold
This face uses a bold variant of the default font, if it has one.
@item italic
This face uses an italic variant of the default font, if it has one.
@item bold-italic
This face uses a bold italic variant of the default font, if it has one.
@item underline
This face underlines text.
@item fixed-pitch
The basic fixed-pitch face.
@item fringe
@cindex fringe
The face for the fringes to the left and right of windows on graphic
displays.  (The fringes are the narrow portions of the Emacs frame
between the text area and the window's right and left borders.)
@item scroll-bar
This face determines the visual appearance of the scroll bar.
@item border
This face determines the color of the frame border.
@item cursor
This face determines the color of the cursor.
@item mouse
This face determines the color of the mouse pointer.
@item tool-bar
This is the basic tool-bar face.  No text appears in the tool bar, but the
colors of this face affect the appearance of tool bar icons.
@item tooltip
This face is used for tooltips.
@item menu
This face determines the colors and font of Emacs's menus.  Setting the
font of LessTif/Motif menus is currently not supported; attempts to set
the font are ignored in this case.
@item trailing-whitespace
The face for highlighting trailing whitespace when
@code{show-trailing-whitespace} is non-nil; see @ref{Trailing Whitespace}.
@item variable-pitch
The basic variable-pitch face.
@end table

@cindex @code{region} face
  When Transient Mark mode is enabled, the text of the region is
highlighted when the mark is active.  This uses the face named
@code{region}; you can control the style of highlighting by changing the
style of this face (@pxref{Face Customization}).  @xref{Transient Mark},
for more information about Transient Mark mode and activation and
deactivation of the mark.

  One easy way to use faces is to turn on Font Lock mode.  This minor
mode, which is always local to a particular buffer, arranges to
choose faces according to the syntax of the text you are editing.  It
can recognize comments and strings in most languages; in several
languages, it can also recognize and properly highlight various other
important constructs.  @xref{Font Lock}, for more information about
Font Lock mode and syntactic highlighting.

  You can print out the buffer with the highlighting that appears
on your screen using the command @code{ps-print-buffer-with-faces}.
@xref{PostScript}.

@node Font Lock
@section Font Lock mode
@cindex Font Lock mode
@cindex mode, Font Lock
@cindex syntax highlighting and coloring

  Font Lock mode is a minor mode, always local to a particular
buffer, which highlights (or ``fontifies'') using various faces
according to the syntax of the text you are editing.  It can
recognize comments and strings in most languages; in several
languages, it can also recognize and properly highlight various other
important constructs---for example, names of functions being defined
or reserved keywords.

@findex font-lock-mode
@findex turn-on-font-lock
  The command @kbd{M-x font-lock-mode} turns Font Lock mode on or off
according to the argument, and toggles the mode when it has no argument.
The function @code{turn-on-font-lock} unconditionally enables Font Lock
mode.  This is useful in mode-hook functions.  For example, to enable
Font Lock mode whenever you edit a C file, you can do this:

@example
(add-hook 'c-mode-hook 'turn-on-font-lock)
@end example

@findex global-font-lock-mode
@vindex global-font-lock-mode
  To turn on Font Lock mode automatically in all modes which support
it, customize the user option @code{global-font-lock-mode} or use the
function @code{global-font-lock-mode} in your @file{.emacs} file, like
this:

@example
(global-font-lock-mode 1)
@end example

  Font Lock mode uses several specifically named faces to do its job,
including @code{font-lock-string-face}, @code{font-lock-comment-face},
and others.  The easiest way to find them all is to use completion
on the face name in @code{set-face-foreground}.

  To change the colors or the fonts used by Font Lock mode to fontify
different parts of text, just change these faces.  There are
two ways to do it:

@itemize @bullet
@item
Invoke @kbd{M-x set-face-foreground} or @kbd{M-x set-face-background}
to change the colors of a particular face used by Font Lock.
@xref{Faces}.  The command @kbd{M-x list-faces-display} displays all
the faces currently known to Emacs, including those used by Font Lock.

@item
Customize the faces interactively with @kbd{M-x customize-face}, as
described in @ref{Face Customization}.
@end itemize

  To get the full benefit of Font Lock mode, you need to choose a
default font which has bold, italic, and bold-italic variants; or else
you need to have a color or gray-scale screen.

@vindex font-lock-maximum-decoration
  The variable @code{font-lock-maximum-decoration} specifies the
preferred level of fontification, for modes that provide multiple
levels.  Level 1 is the least amount of fontification; some modes
support levels as high as 3.  The normal default is ``as high as
possible.''  You can specify an integer, which applies to all modes, or
you can specify different numbers for particular major modes; for
example, to use level 1 for C/C++ modes, and the default level
otherwise, use this:

@example
(setq font-lock-maximum-decoration
      '((c-mode . 1) (c++-mode . 1)))
@end example

@vindex font-lock-maximum-size
  Fontification can be too slow for large buffers, so you can suppress
it.  The variable @code{font-lock-maximum-size} specifies a buffer size,
beyond which buffer fontification is suppressed.

@c @w is used below to prevent a bad page-break.
@vindex font-lock-beginning-of-syntax-function
@cindex incorrect fontification
@cindex parenthesis in column zero and fontification
@cindex brace in column zero and fontification
  Comment and string fontification (or ``syntactic'' fontification)
relies on analysis of the syntactic structure of the buffer text.  For
the sake of speed, some modes, including C mode and Lisp mode,
rely on a special convention: an open-parenthesis or open-brace in the
leftmost column always defines the @w{beginning} of a defun, and is
thus always outside any string or comment.  (@xref{Left Margin
Paren}.)  If you don't follow this convention, Font Lock mode can
misfontify the text that follows an open-parenthesis or open-brace in
the leftmost column that is inside a string or comment.

@cindex slow display during scrolling
  The variable @code{font-lock-beginning-of-syntax-function} (always
buffer-local) specifies how Font Lock mode can find a position
guaranteed to be outside any comment or string.  In modes which use the
leftmost column parenthesis convention, the default value of the variable
is @code{beginning-of-defun}---that tells Font Lock mode to use the
convention.  If you set this variable to @code{nil}, Font Lock no longer
relies on the convention.  This avoids incorrect results, but the price
is that, in some cases, fontification for a changed text must rescan
buffer text from the beginning of the buffer.  This can considerably
slow down redisplay while scrolling, particularly if you are close to
the end of a large buffer.

@findex font-lock-add-keywords
  Font Lock highlighting patterns already exist for many modes, but you
may want to fontify additional patterns.  You can use the function
@code{font-lock-add-keywords}, to add your own highlighting patterns for
a particular mode.  For example, to highlight @samp{FIXME:} words in C
comments, use this:

@example
(font-lock-add-keywords
 'c-mode
 '(("\\<\\(FIXME\\):" 1 font-lock-warning-face t)))
@end example

@findex font-lock-remove-keywords
  To remove keywords from the font-lock highlighting patterns, use the
function @code{font-lock-remove-keywords}.

@cindex just-in-time (JIT) font-lock
@cindex background syntax highlighting
  Fontifying large buffers can take a long time.  To avoid large
delays when a file is visited, Emacs fontifies only the visible
portion of a buffer.  As you scroll through the buffer, each portion
that becomes visible is fontified as soon as it is displayed.  The
parts of the buffer that are not displayed are fontified
``stealthily'', in the background, i.e.@: when Emacs is idle.  You can
control this background fontification, called @dfn{Just-In-Time}, or
@dfn{JIT} Font Lock, by customizing various options in the
customization group @samp{jit-lock}.  @xref{Specific Customization}.

@node Highlight Changes
@section Highlight Changes Mode

@findex highlight-changes-mode
  Use @kbd{M-x highlight-changes-mode} to enable a minor mode
that uses faces (colors, typically) to indicate which parts of
the buffer were changed most recently.

@node Highlight Interactively
@section Interactive Highlighting by Matching
@cindex highlighting by matching
@cindex interactive highlighting

  It is sometimes useful to highlight the strings that match a certain
regular expression.  For example, you might wish to see all the
references to a certain variable in a program source file, or highlight
certain parts in a voluminous output of some program, or make certain
cliches stand out in an article.

@findex hi-lock-mode
  Use the @kbd{M-x hi-lock-mode} command to turn on a minor mode that
allows you to specify regular expressions of the text to be
highlighted.  Hi-lock mode works like Font Lock (@pxref{Font Lock}),
except that it lets you specify explicitly what parts of text to
highlight.  You control Hi-lock mode with these commands:

@table @kbd
@item C-x w h @var{regexp} @key{RET} @var{face} @key{RET}
@kindex C-x w h
@findex highlight-regexp
Highlight text that matches
@var{regexp} using face @var{face} (@code{highlight-regexp}).
By using this command more than once, you can highlight various
parts of the text in different ways.

@item C-x w r @var{regexp} @key{RET}
@kindex C-x w r
@findex unhighlight-regexp
Unhighlight @var{regexp} (@code{unhighlight-regexp}).  You must enter
one of the regular expressions currently specified for highlighting.
(You can use completion, or choose from a menu, to enter one of them
conveniently.)

@item C-x w l @var{regexp} @key{RET} @var{face} @key{RET}
@kindex C-x w l
@findex highlight-lines-matching-regexp
@cindex lines, highlighting
@cindex highlighting lines of text
Highlight entire lines containing a match for @var{regexp}, using face
@var{face} (@code{highlight-lines-matching-regexp}).

@item C-x w b
@kindex C-x w b
@findex hi-lock-write-interactive-patterns
Insert all the current highlighting regexp/face pairs into the buffer
at point, with comment delimiters to prevent them from changing your
program.  This key binding runs the
@code{hi-lock-write-interactive-patterns} command.

These patterns will be read the next time you visit the file while
Hi-lock mode is enabled, or whenever you use the @kbd{M-x
hi-lock-find-patterns} command.

@item C-x w i
@kindex C-x w i
@findex hi-lock-find-patterns
@vindex hi-lock-exclude-modes
Re-read regexp/face pairs in the current buffer
(@code{hi-lock-write-interactive-patterns}).  The list of pairs is
found no matter where in the buffer it may be.

This command does nothing if the major mode is a member of the list
@code{hi-lock-exclude-modes}.
@end table

@node Trailing Whitespace
@section Trailing Whitespace

@cindex trailing whitespace
@cindex whitespace, trailing
@vindex show-trailing-whitespace
  It is easy to leave unnecessary spaces at the end of a line without
realizing it.  In most cases, this @dfn{trailing whitespace} has no
effect, but there are special circumstances where it matters.

  You can make trailing whitespace visible on the screen by setting the
buffer-local variable @code{show-trailing-whitespace} to @code{t}.  Then
Emacs displays trailing whitespace in the face
@code{trailing-whitespace}.

  Trailing whitespace is defined as spaces or tabs at the end of a
line.  But trailing whitespace is not displayed specially if point is
at the end of the line containing the whitespace.  (Doing that looks
ugly while you are typing in new text, and the location of point is
enough in that case to show you that the spaces are present.)

@findex delete-trailing-whitespace
  To delete all trailing whitespace within the current buffer's
restriction (@pxref{Narrowing}), type @kbd{M-x
delete-trailing-whitespace @key{RET}}.  (This command does not remove
the form-feed characters.)

@vindex indicate-empty-lines
@vindex default-indicate-empty-lines
@cindex empty lines
  Emacs can indicate empty lines at the end of the buffer with a
special bitmap on the left fringe of the window.  To enable this
feature, set the buffer-local variable @code{indicate-empty-lines} to
a non-@code{nil} value.  The default value of this variable is
controlled by the variable @code{default-indicate-empty-lines};
by setting that variable, you can enable or disable this feature
for all new buffers.

@node Scrolling
@section Scrolling

  If a buffer contains text that is too large to fit entirely within a
window that is displaying the buffer, Emacs shows a contiguous portion of
the text.  The portion 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.

@table @kbd
@item C-l
Clear screen and redisplay, scrolling the selected window to center
point vertically within it (@code{recenter}).
@item C-v
Scroll forward (a windowful or a specified number of lines) (@code{scroll-up}).
@item @key{NEXT}
@itemx @key{PAGEDOWN}
Likewise, scroll forward.
@item M-v
Scroll backward (@code{scroll-down}).
@item @key{PRIOR}
@itemx @key{PAGEUP}
Likewise, scroll backward.
@item @var{arg} C-l
Scroll so point is on line @var{arg} (@code{recenter}).
@item C-M-l
Scroll heuristically to bring useful information onto the screen
(@code{reposition-window}).
@end table

@kindex C-l
@findex recenter
  The most basic scrolling command is @kbd{C-l} (@code{recenter}) with
no argument.  It clears the entire screen and redisplays all windows.
In addition, it scrolls the selected window so that point is halfway
down from the top of the window.

@kindex C-v
@kindex M-v
@kindex NEXT
@kindex PRIOR
@kindex PAGEDOWN
@kindex PAGEUP
@findex scroll-up
@findex scroll-down
@vindex next-screen-context-lines
  To read the buffer a windowful at a time, use @kbd{C-v}
(@code{scroll-up}) with no argument.  This scrolls forward by nearly
the whole window height.  The effect is to take the two lines at the
bottom of the window and put them at the top, followed by nearly a
whole windowful of lines that were not previously visible.  If point
was in the text that scrolled off the top, it ends up at the new top
of the window.

  @kbd{M-v} (@code{scroll-down}) with no argument scrolls backward in
a similar way, also with 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 2.  The function
keys @key{NEXT} and @key{PRIOR}, or @key{PAGEDOWN} and @key{PAGEUP},
are equivalent to @kbd{C-v} and @kbd{M-v}.

  The commands @kbd{C-v} and @kbd{M-v} with a numeric argument scroll
the text in the selected window up or down a few lines.  @kbd{C-v}
with an argument moves the text and point up, together, that many
lines; it brings the same number of new lines into view at the bottom
of the window.  @kbd{M-v} with numeric argument scrolls the text
downward, bringing that many new lines into view at the top of the
window.  @kbd{C-v} with a negative argument is like @kbd{M-v} and vice
versa.

  The names of scroll commands are based on the direction that the
text moves in the window.  Thus, the command to scroll forward is
called @code{scroll-up} because it moves the text upward on the
screen.  The keys @key{PAGEDOWN} and @key{PAGEUP} derive their names
and customary meanings from a different convention that developed
elsewhere; hence the strange result that @key{PAGEDOWN} runs
@code{scroll-up}.

@vindex scroll-preserve-screen-position
  Some users like the full-screen scroll commands to keep point at the
same screen line.  To enable this behavior, set the variable
@code{scroll-preserve-screen-position} to a non-@code{nil} value.  This
mode is convenient for browsing through a file by scrolling by
screenfuls; if you come back to the screen where you started, point goes
back to the line where it started.  However, this mode is inconvenient
when you move to the next screen in order to move point to the text
there.

  Another way to do scrolling is with @kbd{C-l} with a numeric argument.
@kbd{C-l} does not clear the screen when given an argument; it only scrolls
the selected window.  With a positive argument @var{n}, it repositions text
to put point @var{n} lines down from the top.  An argument of zero puts
point on the very top line.  Point does not move with respect to the text;
rather, the text and point move rigidly on the screen.  @kbd{C-l} with a
negative argument puts point that many lines from the bottom of the window.
For example, @kbd{C-u - 1 C-l} puts point on the bottom line, and @kbd{C-u
- 5 C-l} puts it five lines from the bottom.  @kbd{C-u C-l} scrolls to put
point at the center (vertically) of the selected window.

@kindex C-M-l
@findex reposition-window
  The @kbd{C-M-l} command (@code{reposition-window}) scrolls the current
window heuristically in a way designed to get useful information onto
the screen.  For example, in a Lisp file, this command tries to get the
entire current defun onto the screen if possible.

@vindex scroll-conservatively
  Scrolling happens automatically when point moves out of the visible
portion of the text.  Normally, automatic scrolling centers point
vertically within the window.  However, if you set
@code{scroll-conservatively} to a small number @var{n}, then if you
move point just a little off the screen---less than @var{n}
lines---then Emacs scrolls the text just far enough to bring point
back on screen.  By default, @code{scroll-conservatively} is 0.

@cindex aggressive scrolling
@vindex scroll-up-aggressively
@vindex scroll-down-aggressively 
  When the window does scroll by a longer distance, you can control
how aggressively it scrolls, by setting the variables
@code{scroll-up-aggressively} and @code{scroll-down-aggressively}.
The value of @code{scroll-up-aggressively} should be either
@code{nil}, or a fraction @var{f} between 0 and 1.  A fraction
specifies where on the screen to put point when scrolling upward.
More precisely, when a window scrolls up because point is above the
window start, the new start position is chosen to put point @var{f}
part of the window height from the top.  The larger @var{f}, the more
aggressive the scrolling.

  @code{nil}, which is the default, scrolls to put point at the center.
So it is equivalent to .5.

  Likewise, @code{scroll-down-aggressively} is used for scrolling
down.  The value, @var{f}, specifies how far point should be placed
from the bottom of the window; thus, as with
@code{scroll-up-aggressively}, a larger value is more aggressive.

@vindex scroll-margin
  The variable @code{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 recenters the window.  By default, @code{scroll-margin} is
0.

@node Horizontal Scrolling
@section Horizontal Scrolling
@cindex horizontal scrolling

  @dfn{Horizontal scrolling} means shifting all the lines sideways
within a window---so that some of the text near the left margin is not
displayed at all.  Emacs does this automatically in any window that
uses line truncation rather than continuation: whenever point moves
off the left or right edge of the screen, Emacs scrolls the buffer
horizontally to make point visible.

  When a window has been scrolled horizontally, 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.

  You can use these commands to do explicit horizontal scrolling.

@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

@kindex C-x <
@kindex C-x >
@findex scroll-left
@findex scroll-right
  The command @kbd{C-x <} (@code{scroll-left}) scrolls the selected
window to the left by @var{n} columns with argument @var{n}.  This moves
part of the beginning of each line off the left edge of the window.
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 displayed
normally (with each line starting at the window's left margin);
attempting to do so has no effect.  This means that you don't have to
calculate the argument precisely for @w{@kbd{C-x >}}; any sufficiently large
argument will restore the normal display.

  If you scroll a window horizontally by hand, that sets a lower bound
for automatic horizontal scrolling.  Automatic scrolling will continue
to scroll the window, but never farther to the right than the amount
you previously set by @code{scroll-left}.

@vindex automatic-hscrolling
  To disable automatic horizontal scrolling, set the variable
@code{automatic-hscrolling} to @code{nil}.

@node Follow Mode
@section Follow Mode
@cindex Follow mode
@cindex mode, Follow
@findex follow-mode
@cindex windows, synchronizing
@cindex synchronizing windows

  @dfn{Follow mode} is a minor mode that makes two windows showing the
same buffer scroll as one tall ``virtual window.''  To use Follow mode,
go to a frame with just one window, split it into two side-by-side
windows using @kbd{C-x 3}, and then type @kbd{M-x follow-mode}.  From
then on, you can edit the buffer in either of the two windows, or scroll
either one; the other window follows it.

  In Follow mode, if you move point outside the portion visible in one
window and into the portion visible in the other window, that selects
the other window---again, treating the two as if they were parts of
one large window.

  To turn off Follow mode, type @kbd{M-x follow-mode} a second time.

@node Selective Display
@section Selective Display
@cindex selective display
@findex set-selective-display
@kindex C-x $

  Emacs has the ability to hide lines indented more than a certain number
of columns (you specify how many columns).  You can use this 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}.  Then lines with at least @var{n} columns of
indentation disappear from the screen.  The only indication of their
presence is that three dots (@samp{@dots{}}) appear at the end of each
visible line that is followed by one or more hidden ones.

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

  The hidden lines are still present in the buffer, and most editing
commands see them as usual, so you may find point in the middle of the
hidden 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.

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

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

@node Optional Mode Line
@section Optional Mode Line Features

@cindex line number display
@cindex display of line number
@findex line-number-mode
  The current line number of point appears in the mode line when Line
Number mode is enabled.  Use the command @kbd{M-x line-number-mode} to
turn this mode on and off; normally it is on.  The line number appears
before the buffer percentage @var{pos}, with the letter @samp{L} to
indicate what it is.  @xref{Minor Modes}, for more information about
minor modes and about how to use this command.

@cindex narrowing, and line number display
  If you have narrowed the buffer (@pxref{Narrowing}), the displayed
line number is relative to the accessible portion of the buffer.

@vindex line-number-display-limit
  If the buffer is very large (larger than the value of
@code{line-number-display-limit}), then the line number doesn't appear.
Emacs doesn't compute the line number when the buffer is large, because
that would be too slow.  Set it to @code{nil} to remove the limit.

@vindex line-number-display-limit-width
  Line-number computation can also be slow if the lines in the buffer
are too long.  For this reason, Emacs normally doesn't display line
numbers if the average width, in characters, of lines near point is
larger than the value of the variable
@code{line-number-display-limit-width}.  The default value is 200
characters.

@cindex Column Number mode
@cindex mode, Column Number
@findex column-number-mode
  You can also display the current column number by turning on Column
Number mode.  It displays the current column number preceded by the
letter @samp{C}.  Type @kbd{M-x column-number-mode} to toggle this mode.

@findex display-time
@cindex time (on mode line)
  Emacs can optionally display the time and system load in all mode
lines.  To enable this feature, type @kbd{M-x display-time} or customize
the option @code{display-time-mode}.  The information added to the mode
line usually appears after the buffer name, before the mode names and
their parentheses.  It looks like this:

@example
@var{hh}:@var{mm}pm @var{l.ll}
@end example

@noindent
@vindex display-time-24hr-format
Here @var{hh} and @var{mm} are the hour and minute, followed always by
@samp{am} or @samp{pm}.  @var{l.ll} is the average number of running
processes in the whole system recently.  (Some fields may be missing if
your operating system cannot support them.)  If you prefer time display
in 24-hour format, set the variable @code{display-time-24hr-format}
to @code{t}.

@cindex mail (on mode line)
@vindex display-time-use-mail-icon
@vindex display-time-mail-face
  The word @samp{Mail} appears after the load level if there is mail
for you that you have not read yet.  On a graphical display you can use
an icon instead of @samp{Mail} by customizing
@code{display-time-use-mail-icon}; this may save some space on the mode
line.  You can customize @code{display-time-mail-face} to make the mail
indicator prominent.

@cindex mode line, 3D appearance
@cindex attributes of mode line, changing
@cindex non-integral number of lines in a window
  By default, the mode line is drawn on graphics displays with
3D-style highlighting, like that of a button when it is not being
pressed.  If you don't like this effect, you can disable the 3D
highlighting of the mode line, by customizing the attributes of the
@code{mode-line} face in your @file{.emacs} init file, like this:

@example
(set-face-attribute 'mode-line nil :box nil)
@end example

@noindent
Alternatively, you can turn off the box attribute in your
@file{.Xdefaults} file:

@example
Emacs.mode-line.AttributeBox: off
@end example

@node Text Display
@section How Text Is Displayed
@cindex characters (in text)

  ASCII printing characters (octal codes 040 through 0176) in Emacs
buffers are displayed with their graphics, as are non-ASCII multibyte
printing characters (octal codes above 0400).

  Some ASCII control characters are displayed in special ways.  The
newline character (octal code 012) is displayed by starting a new line.
The tab character (octal code 011) is displayed by moving to the next
tab stop column (normally every 8 columns).

  Other ASCII control characters are normally displayed as a caret
(@samp{^}) followed by the non-control version of the character; thus,
control-A is displayed as @samp{^A}.

  Non-ASCII characters 0200 through 0237 (octal) are displayed with
octal escape sequences; thus, character code 0230 (octal) is displayed
as @samp{\230}.  The display of character codes 0240 through 0377
(octal) may be either as escape sequences or as graphics.  They do not
normally occur in multibyte buffers, but if they do, they are displayed
as Latin-1 graphics.  In unibyte mode, if you enable European display
they are displayed using their graphics (assuming your terminal supports
them), otherwise as escape sequences.  @xref{Single-Byte Character
Support}.

@node Display Custom
@section Customization of Display

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

@vindex mode-line-inverse-video
  The variable @code{mode-line-inverse-video} is an obsolete way of
controlling whether the mode line is displayed in inverse video; the
preferred way of doing this is to change the @code{mode-line} face.
@xref{Mode Line}.  However, if @code{mode-line-inverse-video} has a
value of @code{nil}, then the @code{mode-line} face will be ignored,
and mode-lines will be drawn using the default text face.
@xref{Faces}.

@vindex inverse-video
  If the variable @code{inverse-video} is non-@code{nil}, Emacs attempts
to invert all the lines of the display from what they normally are.

@vindex visible-bell
  If the variable @code{visible-bell} is non-@code{nil}, Emacs attempts
to make the whole screen blink when it would normally make an audible bell
sound.  This variable has no effect if your terminal does not have a way
to make the screen blink.@refill

@vindex no-redraw-on-reenter
  When you reenter Emacs after suspending, Emacs 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 Emacs
is entered and exited, respectively) switch between pages of memory so
as to use one page for Emacs and another page for other output.  Then
you might want to set the variable @code{no-redraw-on-reenter}
non-@code{nil}; this tells Emacs to assume, when resumed, that the
screen page it is using still contains what Emacs 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}, all control characters in
the buffer are displayed with octal escape sequences, except for newline
and tab.  Altering the value of @code{ctl-arrow} makes it local to the
current buffer; until that time, the default value is in effect.  The
default is initially @code{t}.  @xref{Display Tables,, Display Tables,
elisp, The Emacs Lisp Reference Manual}.

@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.  The variable @code{tab-width} must have an
integer value between 1 and 1000, inclusive.

@c @vindex truncate-lines  @c No index entry here, because we have one
@c in the continuation section.
  If the variable @code{truncate-lines} is non-@code{nil}, then each
line of text gets just one screen line for display; if the text line is
too long, display shows only the part that fits.  If
@code{truncate-lines} is @code{nil}, then long text lines display as
more than one screen line, enough to show the whole text of the line.
@xref{Continuation Lines}.  Altering the value of @code{truncate-lines}
makes it local to the current buffer; until that time, the default value
is in effect.  The default is initially @code{nil}.

@c @vindex truncate-partial-width-windows  @c Idx entry is in Split Windows.
  If the variable @code{truncate-partial-width-windows} is
non-@code{nil}, it forces truncation rather than continuation in any
window less than the full width of the screen or frame, regardless of
the value of @code{truncate-lines}.  For information about side-by-side
windows, see @ref{Split Window}.  See also @ref{Display,, Display,
elisp, The Emacs Lisp Reference Manual}.

@vindex baud-rate
  The variable @code{baud-rate} holds the output speed of the
terminal, as far as Emacs knows.  Setting this variable does not
change the speed of actual data transmission, but the value is used
for calculations.  On terminals, it affects padding, and decisions
about whether to scroll part of the screen or redraw it instead.
It also affects the behavior of incremental search.

  On window-systems, @code{baud-rate} is only used to determine how
frequently to look for pending input during display updating.  A
higher value of @code{baud-rate} means that check for pending input
will be done less frequently.

  You can customize the way any particular character code is displayed
by means of a display table.  @xref{Display Tables,, Display Tables,
elisp, The Emacs Lisp Reference Manual}.

@cindex hourglass pointer display
@vindex hourglass-delay
  On a window system, Emacs can optionally display the mouse pointer
in a special shape to say that Emacs is busy.  To turn this feature on
or off, customize the group @code{cursor}.  You can also control the
amount of time Emacs must remain busy before the busy indicator is
displayed, by setting the variable @code{hourglass-delay}.

@findex tty-suppress-bold-inverse-default-colors
  On some text-only terminals, bold face and inverse video together
result in text that is hard to read.  Call the function
@code{tty-suppress-bold-inverse-default-colors} with a non-@code{nil}
argument to suppress the effect of bold-face in this case.

@node Cursor Display
@section Displaying the Cursor

@findex hl-line-mode
@cindex highlight current line
@findex blink-cursor-mode
@cindex cursor, locating visually
@cindex cursor, blinking
  There are a number of ways to customize the display of the cursor.
@kbd{M-x hl-line-mode} enables or disables a minor mode which
highlights the line containing point.  On window systems, the command
@kbd{M-x blink-cursor-mode} turns on or off the blinking of the
cursor.  (On terminals, the terminal itself blinks the cursor, and
Emacs has no control over it.)

  You can customize the cursor's color, and whether it blinks, using
the @code{cursor} Custom group (@pxref{Easy Customization}).

@vindex x-stretch-cursor
@cindex wide block cursor
  When displaying on a window system, Emacs can optionally draw the
block cursor as wide as the character under the cursor---for example,
if the cursor is on a tab character, it would cover the full width
occupied by that tab character.  To enable this feature, set the
variable @code{x-stretch-cursor} to a non-@code{nil} value.

@cindex cursor in non-selected windows
@vindex cursor-in-non-selected-windows
  Normally, the cursor in non-selected windows is shown as a hollow box.
To turn off cursor display in non-selected windows, customize the option
@code{cursor-in-non-selected-windows} to assign it a @code{nil} value.