Compare and round more carefully

[emacs.git] / doc / emacs / windows.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2000-2017 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Windows
@chapter Multiple Windows
@cindex windows in Emacs
@cindex multiple windows in Emacs

  Emacs can split a frame into two or many windows.  Multiple windows
can display parts of different buffers, or different parts of one
buffer.  Multiple frames always imply multiple windows, because each
frame has its own set of windows.  Each window belongs to one and only
one frame.

@menu
* Basic Window::        Introduction to Emacs windows.
* Split Window::        New windows are made by splitting existing windows.
* Other Window::        Moving to another window or doing something to it.
* Pop Up Window::       Finding a file or buffer in another window.
* Change Window::       Deleting windows and changing their sizes.
* Displaying Buffers::  How Emacs picks a window for displaying a buffer.
* Window Convenience::  Convenience functions for window handling.
@end menu

@node Basic Window
@section Concepts of Emacs Windows

  Each Emacs window displays one Emacs buffer at any time.  A single
buffer may appear in more than one window; if it does, any changes in
its text are displayed in all the windows where it appears.  But these
windows can show different parts of the buffer, because each window
has its own value of point.

@cindex selected window
  At any time, one Emacs window is the @dfn{selected window}; the
buffer this window is displaying is the current buffer.  On graphical
displays, the point is indicated by a solid blinking cursor in the
selected window, and by a hollow box in non-selected windows.  On text
terminals, the cursor is drawn only in the selected window.
@xref{Cursor Display}.

  Commands to move point affect the value of point for the selected
Emacs window only.  They do not change the value of point in other
Emacs windows, even those showing the same buffer.  The same is true
for buffer-switching commands such as @kbd{C-x b}; they do not affect
other windows at all.  However, there are other commands such as
@kbd{C-x 4 b} that select a different window and switch buffers in it.
Also, all commands that display information in a window, including
(for example) @kbd{C-h f} (@code{describe-function}) and @kbd{C-x C-b}
(@code{list-buffers}), work by switching buffers in a nonselected
window without affecting the selected window.

  When multiple windows show the same buffer, they can have different
regions, because they can have different values of point.  However,
they all have the same value for the mark, because each buffer has
only one mark position.

  Each window has its own mode line, which displays the buffer name,
modification status and major and minor modes of the buffer that is
displayed in the window.  The selected window's mode line appears in a
different color.  @xref{Mode Line}, for details.

@node Split Window
@section Splitting Windows

@table @kbd
@item C-x 2
Split the selected window into two windows, one above the other
(@code{split-window-below}).
@item C-x 3
Split the selected window into two windows, positioned side by side
(@code{split-window-right}).
@item C-mouse-2
In the mode line of a window, split that window.
@end table

@kindex C-x 2
@findex split-window-below
  @kbd{C-x 2} (@code{split-window-below}) splits the selected window
into two windows, one above the other.  After splitting, the selected
window is the upper one, and the newly split-off window is below.
Both windows have the same value of point as before, and display the
same portion of the buffer (or as close to it as possible).  If
necessary, the windows are scrolled to keep point on-screen.  By
default, the two windows each get half the height of the original
window.  A positive numeric argument specifies how many lines to give
to the top window; a negative numeric argument specifies how many
lines to give to the bottom window.

@vindex split-window-keep-point
  If you change the variable @code{split-window-keep-point} to
@code{nil}, @kbd{C-x 2} instead adjusts the portion of the buffer
displayed by the two windows, as well as the value of point in each
window, in order to keep the text on the screen as close as possible
to what it was before; furthermore, if point was in the lower half of
the original window, the bottom window is selected instead of the
upper one.

@kindex C-x 3
@findex split-window-right
  @kbd{C-x 3} (@code{split-window-right}) splits the selected window
into two side-by-side windows.  The left window is the selected one;
the right window displays the same portion of the same buffer, and has
the same value of point.  A positive numeric argument specifies how
many columns to give the left window; a negative numeric argument
specifies how many columns to give the right window.

@vindex truncate-partial-width-windows
  When you split a window with @kbd{C-x 3}, each resulting window
occupies less than the full width of the frame.  If it becomes too
narrow, the buffer may be difficult to read if continuation lines are
in use (@pxref{Continuation Lines}).  Therefore, Emacs automatically
switches to line truncation if the window width becomes narrower than
50 columns.  This truncation occurs regardless of the value of the
variable @code{truncate-lines} (@pxref{Line Truncation}); it is
instead controlled by the variable
@code{truncate-partial-width-windows}.  If the value of this variable
is a positive integer (the default is 50), that specifies the minimum
total width for a partial-width window before automatic line
truncation occurs; if the value is @code{nil}, automatic line
truncation is disabled; and for any other non-@code{nil} value, Emacs
truncates lines in every partial-width window regardless of its width.
The total width of a window is in column units as reported by
@code{window-total-width} (@pxref{Window Sizes,,, elisp, The Emacs
Lisp Reference Manual}), it includes the fringes, the continuation and
truncation glyphs, the margins, and the scroll bar.

  On text terminals, side-by-side windows are separated by a vertical
divider which is drawn using the @code{vertical-border} face.

@kindex C-mouse-2 @r{(mode line)}
@kindex C-mouse-2 @r{(scroll bar)}
  If you click @kbd{C-mouse-2} in the mode line of a window, that
splits the window, putting a vertical divider where you click.
Depending on how Emacs is compiled, you can also split a window by
clicking @kbd{C-mouse-2} in the scroll bar, which puts a horizontal
divider where you click (this feature does not work when Emacs uses
GTK+ scroll bars).

@vindex window-resize-pixelwise
  By default, when you split a window, Emacs gives each of the
resulting windows dimensions that are an integral multiple of the
default font size of the frame.  That might subdivide the screen
estate unevenly between the resulting windows.  If you set the
variable @code{window-resize-pixelwise} to a non-@code{nil} value,
Emacs will give each window the same number of pixels (give or take
one pixel if the initial dimension was an odd number of pixels).  Note
that when a frame's pixel size is not a multiple of the frame's
character size, at least one window may get resized pixelwise even if
this option is @code{nil}.

@node Other Window
@section Using Other Windows

@table @kbd
@item C-x o
Select another window (@code{other-window}).
@item C-M-v
Scroll the next window (@code{scroll-other-window}).
@item mouse-1
@kbd{mouse-1}, in the text area of a window, selects the window and
moves point to the position clicked.  Clicking in the mode line
selects the window without moving point in it.
@end table

@kindex C-x o
@findex other-window
With the keyboard, you can switch windows by typing @kbd{C-x o}
(@code{other-window}).  That is an @kbd{o}, for ``other'', not a zero.
When there are more than two windows, this command moves through all the
windows in a cyclic order, generally top to bottom and left to right.
After the rightmost and bottommost window, it goes back to the one at
the upper left corner.  A numeric argument means to move several steps
in the cyclic order of windows.  A negative argument moves around the
cycle in the opposite order.  When the minibuffer is active, the
minibuffer is the last window in the cycle; you can switch from the
minibuffer window to one of the other windows, and later switch back and
finish supplying the minibuffer argument that is requested.
@xref{Minibuffer Edit}.

@kindex C-M-v
@findex scroll-other-window
  The usual scrolling commands (@pxref{Display}) apply to the selected
window only, but there is one command to scroll the next window.
@kbd{C-M-v} (@code{scroll-other-window}) scrolls the window that
@kbd{C-x o} would select.  It takes arguments, positive and negative,
like @kbd{C-v}.  (In the minibuffer, @kbd{C-M-v} scrolls the help
window associated with the minibuffer, if any, rather than the next
window in the standard cyclic order; @pxref{Minibuffer Edit}.)

@vindex mouse-autoselect-window
  If you set @code{mouse-autoselect-window} to a non-@code{nil} value,
moving the mouse over a different window selects that window.  This
feature is off by default.

@node Pop Up Window
@section Displaying in Another Window

@cindex selecting buffers in other windows
@kindex C-x 4
  @kbd{C-x 4} is a prefix key for a variety of commands that switch to
a buffer in a different window---either another existing window, or a
new window created by splitting the selected window.  @xref{Window
Choice}, for how Emacs picks or creates the window to use.

@table @kbd
@findex switch-to-buffer-other-window
@item C-x 4 b @var{bufname} @key{RET}
Select buffer @var{bufname} in another window
(@code{switch-to-buffer-other-window}).

@findex display-buffer @r{(command)}
@item C-x 4 C-o @var{bufname} @key{RET}
@kindex C-x 4 C-o
Display buffer @var{bufname} in some window, without trying to select
it (@code{display-buffer}).  @xref{Displaying Buffers}, for details
about how the window is chosen.

@findex find-file-other-window
@item C-x 4 f @var{filename} @key{RET}
Visit file @var{filename} and select its buffer in another window
(@code{find-file-other-window}).  @xref{Visiting}.

@findex dired-other-window
@item C-x 4 d @var{directory} @key{RET}
Select a Dired buffer for directory @var{directory} in another window
(@code{dired-other-window}).  @xref{Dired}.

@c Don't index @kbd{C-x 4 m} and @code{compose-mail-other-window}
@c here, they are indexed in sending.texi, in the "Sending Mail" node.
@item C-x 4 m
Start composing a mail message, similar to @kbd{C-x m} (@pxref{Sending
Mail}), but in another window (@code{compose-mail-other-window}).

@findex find-tag-other-window
@item C-x 4 .
Find the definition of an identifier, similar to @kbd{M-.}
(@pxref{Xref}), but in another window
(@code{xref-find-definitions-other-window}).
@item C-x 4 r @var{filename} @key{RET}
Visit file @var{filename} read-only, and select its buffer in another
window (@code{find-file-read-only-other-window}).  @xref{Visiting}.
@end table

@node Change Window
@section Deleting and Resizing Windows

@cindex delete window
@cindex deleting windows
@table @kbd
@item C-x 0
Delete the selected window (@code{delete-window}).
@item C-x 1
Delete all windows in the selected frame except the selected window
(@code{delete-other-windows}).
@item C-x 4 0
Delete the selected window and kill the buffer that was showing in it
(@code{kill-buffer-and-window}).  The last character in this key
sequence is a zero.
@item C-x ^
Make selected window taller (@code{enlarge-window}).
@item C-x @}
Make selected window wider (@code{enlarge-window-horizontally}).
@item C-x @{
Make selected window narrower (@code{shrink-window-horizontally}).
@item C-x -
Shrink this window if its buffer doesn't need so many lines
(@code{shrink-window-if-larger-than-buffer}).
@item C-x +
Make all windows the same height (@code{balance-windows}).
@end table

@kindex C-x 0
@findex delete-window
  To delete the selected window, type @kbd{C-x 0}
(@code{delete-window}).  (That is a zero.)  Once a window is deleted,
the space that it occupied is given to an adjacent window (but not the
minibuffer window, even if that is active at the time).  Deleting the
window has no effect on the buffer it used to display; the buffer
continues to exist, and you can still switch to it with @kbd{C-x b}.

@findex kill-buffer-and-window
@kindex C-x 4 0
  @kbd{C-x 4 0} (@code{kill-buffer-and-window}) is a stronger command
than @kbd{C-x 0}; it kills the current buffer and then deletes the
selected window.

@kindex C-x 1
@findex delete-other-windows
  @kbd{C-x 1} (@code{delete-other-windows}) deletes all the windows,
@emph{except} the selected one; the selected window expands to use the
whole frame.  (This command cannot be used while the minibuffer window
is active; attempting to do so signals an error.)

@cindex resize window
@cindex resizing windows
@kindex C-x ^
@findex enlarge-window
@kindex C-x @}
@vindex window-min-height
  The command @kbd{C-x ^} (@code{enlarge-window}) makes the selected
window one line taller, taking space from a vertically adjacent window
without changing the height of the frame.  With a positive numeric
argument, this command increases the window height by that many lines;
with a negative argument, it reduces the height by that many lines.
If there are no vertically adjacent windows (i.e., the window is at the
full frame height), that signals an error.  The command also signals
an error if you attempt to reduce the height of any window below a
certain minimum number of lines, specified by the variable
@code{window-min-height} (the default is 4).

@findex enlarge-window-horizontally
@findex shrink-window-horizontally
@vindex window-min-width
  Similarly, @kbd{C-x @}} (@code{enlarge-window-horizontally}) makes
the selected window wider, and @kbd{C-x @{}
(@code{shrink-window-horizontally}) makes it narrower.  These commands
signal an error if you attempt to reduce the width of any window below
a certain minimum number of columns, specified by the variable
@code{window-min-width} (the default is 10).

  Mouse clicks on the mode line (@pxref{Mode Line Mouse}) or on window
dividers (@pxref{Window Dividers}) provide another way to change window
heights and to split or delete windows.

@kindex C-x -
@findex shrink-window-if-larger-than-buffer
  @kbd{C-x -} (@code{shrink-window-if-larger-than-buffer}) reduces the
height of the selected window, if it is taller than necessary to show
the whole text of the buffer it is displaying.  It gives the extra
lines to other windows in the frame.

@kindex C-x +
@findex balance-windows
  You can also use @kbd{C-x +} (@code{balance-windows}) to even out the
heights of all the windows in the selected frame.

@node Displaying Buffers
@section Displaying a Buffer in a Window

  It is a common Emacs operation to display or pop up some buffer
in response to a user command.  There are several different ways in
which commands do this.

  Many commands, like @kbd{C-x C-f} (@code{find-file}), display the
buffer by ``taking over'' the selected window, expecting that the
user's attention will be diverted to that buffer.  These commands
usually work by calling @code{switch-to-buffer} internally
(@pxref{Select Buffer}).

  Some commands try to display intelligently, trying not to take
over the selected window, e.g., by splitting off a new window and
displaying the desired buffer there.  Such commands, which include the
various help commands (@pxref{Help}), work by calling
@code{display-buffer} internally.  @xref{Window Choice}, for details.

  Other commands do the same as @code{display-buffer}, and
additionally select the displaying window so that you can begin
editing its buffer.  The command @kbd{C-x `} (@code{next-error}) is
one example (@pxref{Compilation Mode}).  Such commands work by calling
the function @code{pop-to-buffer} internally.  @xref{Switching
Buffers,,Switching to a Buffer in a Window, elisp, The Emacs Lisp
Reference Manual}.

  Commands with names ending in @code{-other-window} behave like
@code{display-buffer}, except that they never display in the selected
window.  Several of these commands are bound in the @kbd{C-x 4} prefix
key (@pxref{Pop Up Window}).

  Commands with names ending in @code{-other-frame} behave like
@code{display-buffer}, except that they (i) never display in the
selected window and (ii) prefer to create a new frame to display the
desired buffer instead of splitting a window---as though the variable
@code{pop-up-frames} is set to @code{t} (@pxref{Window Choice}).
Several of these commands are bound in the @kbd{C-x 5} prefix key.

@menu
* Window Choice::   How @code{display-buffer} works.
* Temporary Displays::   Displaying non-editable buffers.
@end menu

@node Window Choice
@subsection How @code{display-buffer} works
@findex display-buffer, detailed description

The @code{display-buffer} command (as well as commands that call it
internally) chooses a window to display by following the steps given
below.  @xref{Choosing Window,,Choosing a Window for Display, elisp,
The Emacs Lisp Reference Manual}, for details about how to alter this
sequence of steps.

@itemize
@vindex same-window-buffer-names
@vindex same-window-regexps
@item
First, check if the buffer should be displayed in the selected window
regardless of other considerations.  You can tell Emacs to do this by
adding the desired buffer's name to the list
@code{same-window-buffer-names}, or adding a matching regular
expression to the list @code{same-window-regexps}.  By default, these
variables are @code{nil}, so this step is skipped.

@item
Otherwise, if the buffer is already displayed in an existing window,
reuse that window.  Normally, only windows on the selected frame
are considered, but windows on other frames are also reusable if you
change @code{pop-up-frames} (see below) to @code{t}.

@vindex pop-up-frames
@item
Otherwise, optionally create a new frame and display the buffer there.
By default, this step is skipped.  To enable it, change the variable
@code{pop-up-frames} to a non-@code{nil} value.  The special value
@code{graphic-only} means to do this only on graphical displays.

@item
Otherwise, try to create a new window by splitting a window on the
selected frame, and display the buffer in that new window.

@vindex split-height-threshold
@vindex split-width-threshold
The split can be either vertical or horizontal, depending on the
variables @code{split-height-threshold} and
@code{split-width-threshold}.  These variables should have integer
values.  If @code{split-height-threshold} is smaller than the chosen
window's height, the split puts the new window below.  Otherwise, if
@code{split-width-threshold} is smaller than the window's width, the
split puts the new window on the right.  If neither condition holds,
Emacs tries to split so that the new window is below---but only if the
window was not split before (to avoid excessive splitting).

@item
Otherwise, display the buffer in a window previously showing it.
Normally, only windows on the selected frame are considered, but if
@code{pop-up-frames} is non-@code{nil} the window may be also on another
frame.

@item
Otherwise, display the buffer in an existing window on the selected
frame.

@item
If all the above methods fail for whatever reason, create a new frame
and display the buffer there.
@end itemize

A more advanced and flexible way to customize the behavior of
@code{display-buffer} is by using the option @code{display-buffer-alist}
mentioned in the next section.


@node Temporary Displays
@subsection Displaying non-editable buffers.
@cindex pop-up windows
@cindex temporary windows

Some buffers are shown in windows for perusal rather than for editing.
Help commands (@pxref{Help}) typically use a buffer called @file{*Help*}
for that purpose, minibuffer completion (@pxref{Completion}) uses a
buffer called @file{*Completions*} instead.  Such buffers are usually
displayed only for a short period of time.

  Normally, Emacs chooses the window for such temporary displays via
@code{display-buffer} as described above.  The @file{*Completions*}
buffer, on the other hand, is normally displayed in a window at the
bottom of the selected frame, regardless of the number of windows
already shown on that frame.

  If you prefer Emacs to display a temporary buffer in a different
fashion, we recommend to customize the variable
@code{display-buffer-alist} (@pxref{Choosing Window,,Choosing a Window
for Display, elisp, The Emacs Lisp Reference Manual}).  For example,
to display @file{*Completions*} by splitting a window as described in
the previous section, use the following form in your initialization
file (@pxref{Init File}):

@example
@group
(customize-set-variable
 'display-buffer-alist
 '(("\\*Completions\\*" display-buffer-pop-up-window)))
@end group
@end example

@findex temp-buffer-resize-mode
  The @file{*Completions*} buffer is also special in the sense that
Emacs usually tries to make its window just as large as necessary to
display all of its contents.  To resize windows showing other
temporary displays like, for example, the @file{*Help*} buffer
accordingly, turn on the minor mode (@pxref{Minor Modes})
@code{temp-buffer-resize-mode} (@pxref{Temporary Displays,,Temporary
Displays, elisp, The Emacs Lisp Reference Manual}).

@vindex temp-buffer-max-height
@vindex temp-buffer-max-width
  The maximum size of windows resized by @code{temp-buffer-resize-mode}
can be controlled by customizing the options
@code{temp-buffer-max-height} and @code{temp-buffer-max-width}
(@pxref{Temporary Displays,,Temporary Displays, elisp, The Emacs Lisp
Reference Manual}) and cannot exceed the size of the containing frame.


@node Window Convenience
@section Convenience Features for Window Handling

@findex winner-mode
@cindex Winner mode
@cindex mode, Winner
@cindex undoing window configuration changes
@cindex window configuration changes, undoing
  Winner mode is a global minor mode that records the changes in the
window configuration (i.e., how the frames are partitioned into
windows), so that you can undo them.  You can toggle Winner mode
with @kbd{M-x winner-mode}, or by customizing the variable
@code{winner-mode}.  When the mode is enabled, @kbd{C-c left}
(@code{winner-undo}) undoes the last window configuration change.  If
you change your mind while undoing, you can redo the changes you had
undone using @kbd{C-c right} (@code{M-x winner-redo}).

  Follow mode (@kbd{M-x follow-mode}) synchronizes several windows on
the same buffer so that they always display adjacent sections of that
buffer.  @xref{Follow Mode}.

@cindex Windmove package
@cindex directional window selection
@findex windmove-right
@findex windmove-default-keybindings
  The Windmove package defines commands for moving directionally
between neighboring windows in a frame.  @kbd{M-x windmove-right}
selects the window immediately to the right of the currently selected
one, and similarly for the left, up, and down
counterparts.  @kbd{M-x windmove-default-keybindings} binds these
commands to @kbd{S-right} etc.; doing so disables shift selection for
those keys (@pxref{Shift Selection}).

  The command @kbd{M-x compare-windows} lets you compare the text
shown in different windows.  @xref{Comparing Files}.

@vindex scroll-all-mode
@cindex scrolling windows together
@cindex Scroll-all mode
@cindex mode, Scroll-all
  Scroll All mode (@kbd{M-x scroll-all-mode}) is a global minor mode
that causes scrolling commands and point motion commands to apply to
every single window.