Merge from emacs--rel--22

[gnu-emacs] / doc / emacs / display.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2000, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007, 2008 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.  Many variables also affect the details of
redisplay.  Unless otherwise stated, the variables described in this
chapter have their effect by customizing redisplay itself; therefore,
their values only make a difference at the time of redisplay.

@menu
* Scrolling::              Commands to move text up and down in a window.
* Auto Scrolling::         Redisplay scrolls text automatically when needed.
* Horizontal Scrolling::   Moving text left and right in a window.
* Follow Mode::            Follow mode lets two windows scroll as one.
* Faces::                  How to change the display style using faces.
* Standard Faces::         Emacs' predefined faces.
* Font Lock::              Minor mode for syntactic highlighting using faces.
* Highlight Interactively:: Tell Emacs what text to highlight.
* Fringes::                Enabling or disabling window fringes.
* Displaying Boundaries::  Displaying top and bottom of the buffer.
* Useless Whitespace::     Showing possibly-spurious trailing whitespace.
* Selective Display::      Hiding lines with lots of indentation.
* Optional Mode Line::     Optional mode line display features.
* Text Display::           How text characters are normally displayed.
* Cursor Display::         Features for displaying the cursor.
* Line Truncation::        Truncating lines to fit the screen width instead
                             of continuing them to multiple screen lines.
* Display Custom::         Information on variables for customizing display.
@end menu

@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'' or
``up'' means that text moves up, and new text appears at the bottom.
Scrolling ``backward'' or ``down'' 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 scroll explicitly 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 scrolls the selected window so that point is halfway
down from the top of the window.  On a text terminal, it also clears
the screen and redisplays all windows.  That is useful in case the
screen is garbled (@pxref{Screen Garbled}).

@kindex C-v
@kindex M-v
@kindex NEXT
@kindex PRIOR
@kindex PAGEDOWN
@kindex PAGEUP
@findex scroll-up
@findex scroll-down
  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.

@vindex next-screen-context-lines
  @kbd{M-v} (@code{scroll-down}) with no argument scrolls backward in
a similar way, also with overlap.  The number of lines of overlap that
the @kbd{C-v} or @kbd{M-v} commands leave 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.  In
this mode, when these commands would scroll the text around point off
the screen, or within @code{scroll-margin} lines of the edge, they
move point to keep the same vertical position within the window.
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.

@node Auto Scrolling
@section Automatic Scrolling

@vindex scroll-conservatively
  Redisplay scrolls the buffer automatically when point moves out of
the visible portion of the text.  The purpose of automatic scrolling
is to make point visible, but you can customize many aspects of how
this is done.

  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@tie{}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.  When the text in a window is scrolled horizontally,
text lines are truncated rather than continued (@pxref{Line
Truncation}).  Whenever a window shows truncated lines, Emacs
automatically updates its horizontal scrolling whenever point moves
off the left or right edge of the screen.  You can also 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 use those commands to scroll a window horizontally, 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 hscroll-margin
  The value of the variable @code{hscroll-margin} controls how close
to the window's edges point is allowed to get before the window will
be automatically scrolled.  It is measured in columns.  If the value
is 5, then moving point within 5 columns of the edge causes horizontal
scrolling away from that edge.

@vindex hscroll-step
  The variable @code{hscroll-step} determines how many columns to
scroll the window when point gets too close to the edge.  If it's
zero, horizontal scrolling centers point horizontally within the
window.  If it's a positive integer, it specifies the number of
columns to scroll by.  If it's a floating-point number, it specifies
the fraction of the window's width to scroll by.  The default is zero.

@vindex auto-hscroll-mode
  To disable automatic horizontal scrolling, set the variable
@code{auto-hscroll-mode} 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, both
showing the same buffer, scroll as a single 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 Faces
@section Faces: Controlling Text Display Style
@cindex faces

  You can specify various styles for displaying text using
@dfn{faces}.  Each face can specify various @dfn{face attributes},
such as the font family, the height, weight and slant of the
characters, the foreground and background color, and underlining or
overlining.  A face does not have to specify all of these attributes;
often it inherits most of them from another face.

  On graphical display, all the Emacs face attributes are meaningful.
On a text-only terminal, only some of them work.  Some text-only
terminals support inverse video, bold, and underline attributes; some
support colors.  Text-only terminals generally do not support changing
the height and width or the font family.

  Most major modes assign faces to the text automatically through the
work of Font Lock mode.  @xref{Font Lock}, for more information about
Font Lock mode and syntactic highlighting.  You can print the current
buffer with the highlighting that appears on your screen using the
command @code{ps-print-buffer-with-faces}.  @xref{PostScript}.

  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}.)

  If you want to alter the appearance of all Emacs frames, you need to
customize the frame parameters in the variable
@code{default-frame-alist}; see @ref{Creating Frames,
default-frame-alist}.

  Emacs 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.

@node Standard Faces
@section Standard Faces

@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.  With a prefix argument, this
prompts for a regular expression, and displays only faces with names
matching that regular expression.

  Here are the standard faces for specifying text appearance.  You can
apply them to specific text when you want the effects they produce.

@table @code
@item default
This face is used for ordinary text that doesn't specify any face.
@item bold
This face uses a bold variant of the default font, if it has one.
It's up to you to choose a default font that has a bold variant,
if you want to use 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
This face forces use of a particular fixed-width font.
@item variable-pitch
This face forces use of a particular variable-width font.  It's
reasonable to customize this face to use a different variable-width font,
if you like, but you should not make it a fixed-width font.
@item shadow
This face is used for making the text less noticeable than the surrounding
ordinary text.  Usually this can be achieved by using shades of gray in
contrast with either black or white default foreground color.
@end table

  Here's an incomplete list of faces used to highlight parts of the
text temporarily for specific purposes.  (Many other modes define
their own faces for this purpose.)

@table @code
@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 the current Isearch match.
@item query-replace
This face is used for highlighting the current Query Replace match.
@item lazy-highlight
This face is used for lazy highlighting of Isearch and Query Replace
matches other than the current one.
@item region
This face is used for displaying a selected region (@pxref{Mark}).
@item secondary-selection
This face is used for displaying a secondary X selection (@pxref{Secondary
Selection}).
@item trailing-whitespace
The face for highlighting excess spaces and tabs at the end of a line
when @code{show-trailing-whitespace} is non-@code{nil}; see
@ref{Useless Whitespace}.
@item nobreak-space
The face for displaying the character ``nobreak space.''
@item escape-glyph
The face for highlighting the @samp{\} or @samp{^} that indicates
a control character.  It's also used when @samp{\} indicates a
nobreak space or nobreak (soft) hyphen.
@end table

  These faces control the appearance of parts of the Emacs frame.
They exist as faces to provide a consistent way to customize the
appearance of these parts of the frame.

@table @code
@item mode-line
@itemx modeline
This face is used for the mode line of the currently selected window,
and for menu bars when toolkit menus are not used.  By default, it's
drawn with shadows for a ``raised'' effect on graphical displays, and
drawn as the inverse of the default face on non-windowed terminals.
@code{modeline} is an alias for the @code{mode-line} face, for
compatibility with old Emacs versions.
@item mode-line-inactive
Like @code{mode-line}, but used for mode lines of the windows other
than the selected one (if @code{mode-line-in-non-selected-windows} is
non-@code{nil}).  This face inherits from @code{mode-line}, so changes
in that face affect mode lines in all windows.
@item mode-line-highlight
Like @code{highlight}, but used for portions of text on mode lines.
@item mode-line-buffer-id
This face is used for buffer identification parts in the mode line.
@item header-line
Similar to @code{mode-line} for a window's header line, which appears
at the top of a window just as the mode line appears at the bottom.
Most windows do not have a header line---only some special modes, such
Info mode, create one.
@item vertical-border
This face is used for the vertical divider between windows.
By default this face inherits from the @code{mode-line-inactive} face
on character terminals.  On graphical displays the foreground color of
this face is used for the vertical line between windows without
scrollbars.
@item minibuffer-prompt
@cindex @code{minibuffer-prompt} face
@vindex minibuffer-prompt-properties
This face is used for the prompt strings displayed in the minibuffer.
By default, Emacs automatically adds this face to the value of
@code{minibuffer-prompt-properties}, which is a list of text
properties used to display the prompt text.  (This variable takes
effect when you enter the minibuffer.)
@item fringe
@cindex @code{fringe} face
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.)
@xref{Fringes}.
@item scroll-bar
This face determines the visual appearance of the scroll bar.
@xref{Scroll Bars}.
@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 face determines the color of tool bar icons.  @xref{Tool Bars}.
@item tooltip
This face is used for tooltips.  @xref{Tooltips}.
@item menu
@cindex menu bar appearance
@cindex @code{menu} face, no effect if customized
@cindex customization of @code{menu} face
This face determines the colors and font of Emacs's menus.  @xref{Menu
Bars}.  Setting the font of LessTif/Motif menus is currently not
supported; attempts to set the font are ignored in this case.
Likewise, attempts to customize this face in Emacs built with GTK and
in the MS-Windows/Mac ports are ignored by the respective GUI toolkits;
you need to use system-wide styles and options to change the
appearance of the menus.
@end table

@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'') the buffer contents 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.
Some special modes, such as Occur mode and Info mode, have completely
specialized ways of assigning fonts for Font Lock mode.

@findex font-lock-mode
  Font Lock mode is turned on by default in all modes which support it.
You can toggle font-lock for each buffer with the command @kbd{M-x
font-lock-mode}.  Using a positive argument unconditionally turns Font
Lock mode on, and a negative or zero argument turns it off.

@findex global-font-lock-mode
@vindex global-font-lock-mode
  If you do not wish Font Lock mode to be turned on by default,
customize the variable @code{global-font-lock-mode} using the Customize
interface (@pxref{Easy Customization}), or use the function
@code{global-font-lock-mode} in your @file{.emacs} file, like this:

@example
(global-font-lock-mode 0)
@end example

@noindent
This variable, like all the variables that control Font Lock mode,
take effect whenever fontification is done; that is, potentially at
any time.

@findex turn-on-font-lock
  If you have disabled Global Font Lock mode, you can still enable Font
Lock for specific major modes by adding the function
@code{turn-on-font-lock} to the mode hooks (@pxref{Hooks}).  For
example, to enable Font Lock mode for editing C files, you can do this:

@example
(add-hook 'c-mode-hook 'turn-on-font-lock)
@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 @kbd{M-x
customize-group @key{RET} font-lock-faces @key{RET}}.  You can then
use that customization buffer to customize the appearance of these
faces.  @xref{Face Customization}.

  You can also customize these faces using @kbd{M-x
set-face-foreground} or @kbd{M-x set-face-background}.  @xref{Faces}.

@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 for buffers above a certain size.  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 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}.  @xref{Search-based
Fontification,,, elisp, The Emacs Lisp Reference Manual}, for
documentation of the format of this list.

@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, also called @dfn{Just-In-Time}
(or @dfn{JIT}) Lock, by customizing variables in the customization
group @samp{jit-lock}.  @xref{Specific Customization}.

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

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

@cindex Hi Lock mode
@findex hi-lock-mode
  Hi Lock mode highlights text that matches regular expressions you
specify.  For example, you might wish to see all the references to a
certain variable in a program source file, highlight certain parts in
a voluminous output of some program, or make certain names stand out
in an article.  Use the @kbd{M-x hi-lock-mode} command to enable (or
disable) Hi Lock mode.  To enable Hi Lock mode for all buffers, use
@kbd{M-x global-hi-lock-mode} or place @code{(global-hi-lock-mode 1)}
in your @file{.emacs} file.

  Hi Lock mode works like Font Lock mode (@pxref{Font Lock}), except
that you specify explicitly the regular expressions to highlight.  You
control them 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}).  The highlighting will remain as long as
the buffer is loaded.  For example, to highlight all occurrences of
the word ``whim'' using the default face (a yellow background)
@kbd{C-x w h whim @key{RET} @key{RET}}.  Any face can be used for
highlighting, Hi Lock provides several of its own and these are
pre-loaded into a history list.  While being prompted for a face use
@kbd{M-p} and @kbd{M-n} to cycle through them.

You can use this command multiple times, specifying various regular
expressions to highlight 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}).

If you invoke this from the menu, you select the expression to
unhighlight from a list.  If you invoke this from the keyboard, you
use the minibuffer.  It will show the most recently added regular
expression; use @kbd{M-p} to show the next older expression and
@kbd{M-n} to select the next newer expression.  (You can also type the
expression by hand, with completion.)  When the expression you want to
unhighlight appears in the minibuffer, press @kbd{@key{RET}} to exit
the minibuffer and unhighlight it.

@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 are extracted from the comments, if appropriate, if you
invoke @kbd{M-x hi-lock-find-patterns}, or if you visit the file while
Hi Lock mode is enabled (since that runs @code{hi-lock-find-patterns}).

@item C-x w i
@kindex C-x w i
@findex hi-lock-find-patterns
Extract regexp/face pairs from comments in the current buffer
(@code{hi-lock-find-patterns}).  Thus, you can enter patterns
interactively with @code{highlight-regexp}, store them into the file
with @code{hi-lock-write-interactive-patterns}, edit them (perhaps
including different faces for different parenthesized parts of the
match), and finally use this command (@code{hi-lock-find-patterns}) to
have Hi Lock highlight the edited patterns.

@vindex hi-lock-file-patterns-policy
The variable @code{hi-lock-file-patterns-policy} controls whether Hi
Lock mode should automatically extract and highlight patterns found in
a file when it is visited.  Its value can be @code{nil} (never
highlight), @code{t} (highlight the patterns), @code{ask} (query the
user), or a function.  If it is a function,
@code{hi-lock-find-patterns} calls it with the patterns as argument;
if the function returns non-@code{nil}, the patterns are used.  The
default is @code{nil}.  Note that patterns are always highlighted if
you call @code{hi-lock-find-patterns} directly, regardless of the
value of this variable.

@vindex hi-lock-exclude-modes
Also, @code{hi-lock-find-patterns} does nothing if the current major
mode's symbol is a member of the list @code{hi-lock-exclude-modes}.
@end table

@node Fringes
@section Window Fringes
@cindex fringes

  On a graphical display, each Emacs window normally has narrow
@dfn{fringes} on the left and right edges.  The fringes display
indications about the text in the window.

  The most common use of the fringes is to indicate a continuation
line, when one line of text is split into multiple lines on the
screen.  The left fringe shows a curving arrow for each screen line
except the first, indicating that ``this is not the real beginning.''
The right fringe shows a curving arrow for each screen line except the
last, indicating that ``this is not the real end.''

  The fringes indicate line truncation with short horizontal arrows
meaning ``there's more text on this line which is scrolled
horizontally out of view;'' clicking the mouse on one of the arrows
scrolls the display horizontally in the direction of the arrow.   The
fringes can also indicate other things, such as empty lines, or where a
program you are debugging is executing (@pxref{Debuggers}).

@findex set-fringe-style
@findex fringe-mode
  You can enable and disable the fringes for all frames using
@kbd{M-x fringe-mode}.  To enable and disable the fringes
for the selected frame, use @kbd{M-x set-fringe-style}.

@node Displaying Boundaries
@section Displaying Boundaries

@vindex indicate-buffer-boundaries
  On a graphical display, Emacs can indicate the buffer boundaries in
the fringes.  It indicates the first line and the last line with
angle images in the fringes.  This can be combined with up and down
arrow images which say whether it is possible to scroll the window up
and down.

  The buffer-local variable @code{indicate-buffer-boundaries} controls
how the buffer boundaries and window scrolling is indicated in the
fringes.  If the value is @code{left} or @code{right}, both angle and
arrow bitmaps are displayed in the left or right fringe, respectively.

  If value is an alist, each element @code{(@var{indicator} .
@var{position})} specifies the position of one of the indicators.
The @var{indicator} must be one of @code{top}, @code{bottom},
@code{up}, @code{down}, or @code{t} which specifies the default
position for the indicators not present in the alist.
The @var{position} is one of @code{left}, @code{right}, or @code{nil}
which specifies not to show this indicator.

  For example, @code{((top . left) (t . right))} places the top angle
bitmap in left fringe, the bottom angle bitmap in right fringe, and
both arrow bitmaps in right fringe.  To show just the angle bitmaps in
the left fringe, but no arrow bitmaps, use @code{((top .  left)
(bottom . left))}.

@vindex default-indicate-buffer-boundaries
  The value of the variable @code{default-indicate-buffer-boundaries}
is the default value for @code{indicate-buffer-boundaries} in buffers
that do not override it.

@node Useless Whitespace
@section Useless Whitespace

@cindex trailing whitespace
@cindex whitespace, trailing
@vindex show-trailing-whitespace
  It is easy to leave unnecessary spaces at the end of a line, or
empty lines at the end of a file, without realizing it.  In most
cases, this @dfn{trailing whitespace} has no effect, but there are
special circumstances where it matters.  It can also be a nuisance
that the line has ``changed,'' when the change is just spaces added or
removed at the end.

  You can make trailing whitespace at the end of a line 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}.

  This feature does not apply when point is at the end of the line
containing the whitespace.  Strictly speaking, that is ``trailing
whitespace'' nonetheless, but displaying it specially in that case
looks ugly while you are typing in new text.  In this special case,
the location of point is enough to show you that the spaces are
present.

@findex delete-trailing-whitespace
  To delete all trailing whitespace within the current buffer's
accessible portion (@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 unused lines
@cindex fringes, and unused line indication
  Emacs can indicate unused lines at the end of the window with a
small image in the left fringe (@pxref{Fringes}).  The image appears
for window lines that do not correspond to any buffer text.  Blank
lines at the end of the buffer then stand out because they do not have
this image in the fringe.

  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.  (This feature
currently doesn't work on text-only terminals.)

@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 in the current buffer, 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.

  See also @ref{Outline Mode} for another way to hide part of
the text in a buffer.

@node Optional Mode Line
@section Optional Mode Line Features

@cindex buffer size display
@cindex display of buffer size
@findex size-indication-mode
  The buffer percentage @var{pos} indicates the percentage of the
buffer above the top of the window.  You can additionally display the
size of the buffer by typing @kbd{M-x size-indication-mode} to turn on
Size Indication mode.  The size will be displayed immediately
following the buffer percentage like this:

@example
@var{POS} of @var{SIZE}
@end example

@noindent
Here @var{SIZE} is the human readable representation of the number of
characters in the buffer, which means that @samp{k} for 10^3, @samp{M}
for 10^6, @samp{G} for 10^9, etc., are used to abbreviate.

@cindex narrowing, and buffer size display
  If you have narrowed the buffer (@pxref{Narrowing}), the size of the
accessible part of the buffer is shown.

@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
after the buffer percentage @var{pos}, with the letter @samp{L} to
indicate what it is.

@cindex Column Number mode
@cindex mode, Column Number
@findex column-number-mode
  Similarly, you can display the current column number by turning on
Column number mode with @kbd{M-x column-number-mode}.  The column
number is indicated by the letter @samp{C}.  However, when both of
these modes are enabled, the line and column numbers are displayed in
parentheses, the line number first, rather than with @samp{L} and
@samp{C}.  For example: @samp{(561,2)}.  @xref{Minor Modes}, for more
information about minor modes and about how to use these commands.

@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.
Thus, it isn't suitable as an argument to @code{goto-line}.  (Use
@code{what-line} command to see the line number relative to the whole
file.)

@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.

@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
@vindex display-time-mail-file
@vindex display-time-mail-directory
  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.  Use @code{display-time-mail-file} to specify
the mail file to check, or set @code{display-time-mail-directory}
to specify the directory to check for incoming mail (any nonempty regular
file in the directory is considered as ``newly arrived mail'').

@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.  @xref{Face Customization}.

@cindex non-selected windows, mode line appearance
  By default, the mode line of nonselected windows is displayed in a
different face, called @code{mode-line-inactive}.  Only the selected
window is displayed in the @code{mode-line} face.  This helps show
which window is selected.  When the minibuffer is selected, since
it has no mode line, the window from which you activated the minibuffer
has its mode line displayed using @code{mode-line}; as a result,
ordinary entry to the minibuffer does not change any mode lines.

@vindex mode-line-in-non-selected-windows
  You can disable use of @code{mode-line-inactive} by setting variable
@code{mode-line-in-non-selected-windows} to @code{nil}; then all mode
lines are displayed in the @code{mode-line} face.

@vindex eol-mnemonic-unix
@vindex eol-mnemonic-dos
@vindex eol-mnemonic-mac
@vindex eol-mnemonic-undecided
  You can customize the mode line display for each of the end-of-line
formats by setting each of the variables @code{eol-mnemonic-unix},
@code{eol-mnemonic-dos}, @code{eol-mnemonic-mac}, and
@code{eol-mnemonic-undecided} to the strings you prefer.

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

  @acronym{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 @acronym{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 @acronym{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}.  The caret appears in face
@code{escape-glyph}.

  Non-@acronym{ASCII} characters 0200 through 0237 (octal) are
displayed with octal escape sequences; thus, character code 0230
(octal) is displayed as @samp{\230}.  The backslash appears in face
@code{escape-glyph}.

@vindex ctl-arrow
  If the variable @code{ctl-arrow} is @code{nil}, 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}.

  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{Unibyte Mode}.

@vindex nobreak-char-display
@cindex no-break space, display
@cindex no-break hyphen, display
@cindex soft hyphen, display
  Some character sets define ``no-break'' versions of the space and
hyphen characters, which are used where a line should not be broken.
Emacs normally displays these characters with special faces
(respectively, @code{nobreak-space} and @code{escape-glyph}) to
distinguish them from ordinary spaces and hyphens.  You can turn off
this feature by setting the variable @code{nobreak-char-display} to
@code{nil}.  If you set the variable to any other value, that means to
prefix these characters with an escape character.

@vindex tab-width
@vindex default-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.  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.  The variable
@code{default-tab-width} controls the default value of this variable
for buffers where you have not set it locally.

  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}.

@node Cursor Display
@section Displaying the Cursor

@findex blink-cursor-mode
@vindex blink-cursor-alist
@cindex cursor, locating visually
@cindex cursor, blinking
  You can customize the cursor's color, and whether it blinks, using
the @code{cursor} Custom group (@pxref{Easy Customization}).  On
a graphical display, the command @kbd{M-x blink-cursor-mode} enables
or disables the blinking of the cursor.  (On text terminals, the
terminal itself blinks the cursor, and Emacs has no control over it.)
You can control how the cursor appears when it blinks off by setting
the variable @code{blink-cursor-alist}.

@vindex visible-cursor
  Some text terminals offer two different cursors: the normal cursor
and the very visible cursor, where the latter may be e.g. bigger or
blinking.  By default Emacs uses the very visible cursor, and switches
to it when you start or resume Emacs.  If the variable
@code{visible-cursor} is @code{nil} when Emacs starts or resumes, it
doesn't switch, so it uses the normal cursor.

@cindex cursor in non-selected windows
@vindex cursor-in-non-selected-windows
  Normally, the cursor appears in non-selected windows without
blinking, with the same appearance as when the blinking cursor blinks
``off.''  For a box cursor, this is a hollow box; for a bar cursor,
this is a thinner bar.  To turn off cursors in non-selected windows,
customize the variable @code{cursor-in-non-selected-windows} and
assign it a @code{nil} value.

@vindex x-stretch-cursor
@cindex wide block cursor
  On graphical displays, 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.

@findex hl-line-mode
@findex global-hl-line-mode
@cindex highlight current line
  To make the cursor even more visible, you can use HL Line mode, a
minor mode that highlights the line containing point.  Use @kbd{M-x
hl-line-mode} to enable or disable it in the current buffer.  @kbd{M-x
global-hl-line-mode} enables or disables the same mode globally.

@node Line Truncation
@section Truncation of Lines

@cindex truncation
@cindex line truncation, and fringes
  As an alternative to continuation, Emacs can display long lines by
@dfn{truncation}.  This means that all the characters that do not fit
in the width of the screen or window do not appear at all.  On
graphical displays, a small straight arrow in the fringe indicates
truncation at either end of the line.  On text-only terminals, @samp{$}
appears 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.

@vindex truncate-lines
@findex toggle-truncate-lines
  Horizontal scrolling automatically causes line truncation
(@pxref{Horizontal Scrolling}).  You can explicitly enable line
truncation for a particular buffer with the command @kbd{M-x
toggle-truncate-lines}.  This works by locally changing the variable
@code{truncate-lines}.  If that variable is non-@code{nil}, long lines
are truncated; if it is @code{nil}, they are continued onto multiple
screen lines.  Setting the variable @code{truncate-lines} in any way
makes it local to the current buffer; until that time, the default
value is in effect.  The default value is normally @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 overflow-newline-into-fringe
  If the variable @code{overflow-newline-into-fringe} is
non-@code{nil} on a graphical display, then Emacs does not continue or
truncate a line which is exactly as wide as the window.  Instead, the
newline overflows into the right fringe, and the cursor appears in the
fringe when positioned on that newline.

@node Display Custom
@section Customization of Display

  This section describes variables (@pxref{Variables}) that you can
change to customize how Emacs displays.  Beginning users can skip
it.
@c the reason for that pxref is because an xref early in the
@c ``echo area'' section leads here.

@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.

@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.  The value takes effect when
there is someting to echo.  @xref{Echo Area}.

@vindex baud-rate
  The variable @anchor{baud-rate}@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 text-only 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 graphical displays, @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.

@cindex hourglass pointer display
@vindex hourglass-delay
  On graphical display, 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}.

@vindex overline-margin
  On graphical display, the variable @code{overline-margin} specifies the vertical position
of an overline above the text, including the height of the overline
itself (1 pixel).  The default value is 2 pixels.

@vindex x-underline-at-descent-line
  On graphical display, Emacs normally draws an underline at the
baseline level of the font.  If @code{x-underline-at-descent-line} is
non-@code{nil}, Emacs draws the underline at the same height as the
font's descent line.

@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.

@vindex no-redraw-on-reenter
  On a text-only terminal, 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.  On such terminals, 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.

@ignore
   arch-tag: 2219f910-2ff0-4521-b059-1bd231a536c4
@end ignore