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

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-1999, 2001-2012
@c   Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Windows
@chapter Windows

This chapter describes the functions and variables related to Emacs
windows.  @xref{Frames}, for how windows are assigned an area of screen
available for Emacs to use.  @xref{Display}, for information on how text
is displayed in windows.

@menu
* Basic Windows::           Basic information on using windows.
* Windows and Frames::      Relating windows to the frame they appear on.
* Window Sizes::            Accessing a window's size.
* Resizing Windows::        Changing the sizes of windows.
* Splitting Windows::       Creating a new window.
* Deleting Windows::        Removing a window from its frame.
* Recombining Windows::     Preserving the frame layout when splitting and
                              deleting windows.
* Selecting Windows::       The selected window is the one that you edit in.
* Cyclic Window Ordering::  Moving around the existing windows.
* Buffers and Windows::     Each window displays the contents of a buffer.
* Switching Buffers::       Higher-level functions for switching to a buffer.
* Choosing Window::         How to choose a window for displaying a buffer.
* Display Action Functions:: Subroutines for @code{display-buffer}.
* Choosing Window Options:: Extra options affecting how buffers are displayed.
* Window History::          Each window remembers the buffers displayed in it.
* Dedicated Windows::       How to avoid displaying another buffer in
                              a specific window.
* Quitting Windows::        How to restore the state prior to displaying a
                              buffer.
* Window Point::            Each window has its own location of point.
* Window Start and End::    Buffer positions indicating which text is
                              on-screen in a window.
* Textual Scrolling::       Moving text up and down through the window.
* Vertical Scrolling::      Moving the contents up and down on the window.
* Horizontal Scrolling::    Moving the contents sideways on the window.
* Coordinates and Windows:: Converting coordinates to windows.
* Window Configurations::   Saving and restoring the state of the screen.
* Window Parameters::       Associating additional information with windows.
* Window Hooks::            Hooks for scrolling, window size changes,
                              redisplay going past a certain point,
                              or window configuration changes.
@end menu


@node Basic Windows
@section Basic Concepts of Emacs Windows
@cindex window

A @dfn{window} is a area of the screen that is used to display a
buffer (@pxref{Buffers}).  In Emacs Lisp, windows are represented by a
special Lisp object type.

@cindex multiple windows
  Windows are grouped into frames (@pxref{Frames}).  Each frame
contains at least one window; the user can subdivide it into multiple,
non-overlapping windows to view several buffers at once.  Lisp
programs can use multiple windows for a variety of purposes.  In
Rmail, for example, you can view a summary of message titles in one
window, and the contents of the selected message in another window.

@cindex terminal screen
@cindex screen of terminal
  Emacs uses the word ``window'' with a different meaning than in
graphical desktop environments and window systems, such as the X
Window System.  When Emacs is run on X, each of its graphical X
windows is an Emacs frame (containing one or more Emacs windows).
When Emacs is run on a text terminal, the frame fills the entire
terminal screen.

@cindex tiled windows
  Unlike X windows, Emacs windows are @dfn{tiled}; they never overlap
within the area of the frame.  When a window is created, resized, or
deleted, the change in window space is taken from or given to the
adjacent windows, so that the total area of the frame is unchanged.

@defun windowp object
This function returns @code{t} if @var{object} is a window (whether or
not it displays a buffer).  Otherwise, it returns @code{nil}.
@end defun

@cindex live windows
A @dfn{live window} is one that is actually displaying a buffer in a
frame.

@defun window-live-p object
This function returns @code{t} if @var{object} is a live window and
@code{nil} otherwise.  A live window is one that displays a buffer.
@end defun

@cindex internal windows
The windows in each frame are organized into a @dfn{window tree}.
@xref{Windows and Frames}.  The leaf nodes of each window tree are live
windows---the ones actually displaying buffers.  The internal nodes of
the window tree are @dfn{internal windows}, which are not live.

@cindex valid windows
   A @dfn{valid window} is one that is either live or internal.  A valid
window can be @dfn{deleted}, i.e. removed from its frame
(@pxref{Deleting Windows}); then it is no longer valid, but the Lisp
object representing it might be still referenced from other Lisp
objects.  A deleted window may be made valid again by restoring a saved
window configuration (@pxref{Window Configurations}).

   You can distinguish valid windows from deleted windows with
@code{window-valid-p}.

@defun window-valid-p object
This function returns @code{t} if @var{object} is a live window, or an
internal window in a window tree.  Otherwise, it returns @code{nil},
including for the case where @var{object} is a deleted window.
@end defun

@cindex selected window
@cindex window selected within a frame
  In each frame, at any time, exactly one Emacs window is designated
as @dfn{selected within the frame}.  For the selected frame, that
window is called the @dfn{selected window}---the one in which most
editing takes place, and in which the cursor for selected windows
appears (@pxref{Cursor Parameters}).  The selected window's buffer is
usually also the current buffer, except when @code{set-buffer} has
been used (@pxref{Current Buffer}).  As for non-selected frames, the
window selected within the frame becomes the selected window if the
frame is ever selected.  @xref{Selecting Windows}.

@defun selected-window
This function returns the selected window (which is always a live
window).
@end defun

@node Windows and Frames
@section Windows and Frames

Each window belongs to exactly one frame (@pxref{Frames}).

@defun window-frame window
This function returns the frame that the window @var{window} belongs
to.  If @var{window} is @code{nil}, it defaults to the selected
window.
@end defun

@defun window-list &optional frame minibuffer window
This function returns a list of live windows belonging to the frame
@var{frame}.  If @var{frame} is omitted or @code{nil}, it defaults to
the selected frame.

The optional argument @var{minibuffer} specifies whether to include
the minibuffer window in the returned list.  If @var{minibuffer} is
@code{t}, the minibuffer window is included.  If @var{minibuffer} is
@code{nil} or omitted, the minibuffer window is included only if it is
active.  If @var{minibuffer} is neither @code{nil} nor @code{t}, the
minibuffer window is never included.

The optional argument @var{window}, if non-@code{nil}, should be a live
window on the specified frame; then @var{window} will be the first
element in the returned list.  If @var{window} is omitted or @code{nil},
the window selected within the frame is the first element.
@end defun

@cindex window tree
@cindex root window
  Windows in the same frame are organized into a @dfn{window tree},
whose leaf nodes are the live windows.  The internal nodes of a window
tree are not live; they exist for the purpose of organizing the
relationships between live windows.  The root node of a window tree is
called the @dfn{root window}.  It can be either a live window (if the
frame has just one window), or an internal window.

  A minibuffer window (@pxref{Minibuffer Windows}) is not part of its
frame's window tree unless the frame is a minibuffer-only frame.
Nonetheless, most of the functions in this section accept the
minibuffer window as an argument.  Also, the function
@code{window-tree} described at the end of this section lists the
minibuffer window alongside the actual window tree.

@defun frame-root-window &optional frame-or-window
This function returns the root window for @var{frame-or-window}.  The
argument @var{frame-or-window} should be either a window or a frame;
if omitted or @code{nil}, it defaults to the selected frame.  If
@var{frame-or-window} is a window, the return value is the root window
of that window's frame.
@end defun

@cindex parent window
@cindex child window
@cindex sibling window
  When a window is split, there are two live windows where previously
there was one.  One of these is represented by the same Lisp window
object as the original window, and the other is represented by a
newly-created Lisp window object.  Both of these live windows become
leaf nodes of the window tree, as @dfn{child windows} of a single
internal window.  If necessary, Emacs automatically creates this
internal window, which is also called the @dfn{parent window}, and
assigns it to the appropriate position in the window tree.  A set of
windows that share the same parent are called @dfn{siblings}.

@cindex parent window
@defun window-parent &optional window
This function returns the parent window of @var{window}.  If
@var{window} is omitted or @code{nil}, it defaults to the selected
window.  The return value is @code{nil} if @var{window} has no parent
(i.e. it is a minibuffer window or the root window of its frame).
@end defun

  Each internal window always has at least two child windows.  If this
number falls to one as a result of window deletion, Emacs
automatically deletes the internal window, and its sole remaining
child window takes its place in the window tree.

  Each child window can be either a live window, or an internal window
(which in turn would have its own child windows).  Therefore, each
internal window can be thought of as occupying a certain rectangular
@dfn{screen area}---the union of the areas occupied by the live
windows that are ultimately descended from it.

@cindex window combination
@cindex vertical combination
@cindex horizontal combination
  For each internal window, the screen areas of the immediate children
are arranged either vertically or horizontally (never both).  If the
child windows are arranged one above the other, they are said to form
a @dfn{vertical combination}; if they are arranged side by side, they
are said to form a @dfn{horizontal combination}.  Consider the
following example:

@smallexample
@group
     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|

@end group
@end smallexample

@noindent
The root window of this frame is an internal window, @code{W1}.  Its
child windows form a horizontal combination, consisting of the live
window @code{W2} and the internal window @code{W3}.  The child windows
of @code{W3} form a vertical combination, consisting of the live
windows @code{W4} and @code{W5}.  Hence, the live windows in this
window tree are @code{W2} @code{W4}, and @code{W5}.

  The following functions can be used to retrieve a child window of an
internal window, and the siblings of a child window.

@defun window-top-child window
This function returns the topmost child window of @var{window}, if
@var{window} is an internal window whose children form a vertical
combination.  For any other type of window, the return value is
@code{nil}.
@end defun

@defun window-left-child window
This function returns the leftmost child window of @var{window}, if
@var{window} is an internal window whose children form a horizontal
combination.  For any other type of window, the return value is
@code{nil}.
@end defun

@defun window-child window
This function returns the first child window of the internal window
@var{window}---the topmost child window for a vertical combination, or
the leftmost child window for a horizontal combination.  If
@var{window} is a live window, the return value is @code{nil}.
@end defun

@defun window-combined-p &optional window horizontal
This function returns a non-@code{nil} value if and only if
@var{window} is part of a vertical combination.  If @var{window} is
omitted or @code{nil}, it defaults to the selected one.

If the optional argument @var{horizontal} is non-@code{nil}, this
means to return non-@code{nil} if and only if @var{window} is part of
a horizontal combination.
@end defun

@defun window-next-sibling &optional window
This function returns the next sibling of the window @var{window}.  If
omitted or @code{nil}, @var{window} defaults to the selected window.
The return value is @code{nil} if @var{window} is the last child of
its parent.
@end defun

@defun window-prev-sibling &optional window
This function returns the previous sibling of the window @var{window}.
If omitted or @code{nil}, @var{window} defaults to the selected
window.  The return value is @code{nil} if @var{window} is the first
child of its parent.
@end defun

The functions @code{window-next-sibling} and
@code{window-prev-sibling} should not be confused with the functions
@code{next-window} and @code{previous-window}, which return the next
and previous window, respectively, in the cyclic ordering of windows
(@pxref{Cyclic Window Ordering}).

  You can use the following functions to find the first live window on
a frame, and to retrieve the entire window tree of a frame:

@defun frame-first-window &optional frame-or-window
This function returns the live window at the upper left corner of the
frame specified by @var{frame-or-window}.  The argument
@var{frame-or-window} must denote a window or a live frame and defaults
to the selected frame.  If @var{frame-or-window} specifies a window,
this function returns the first window on that window's frame.  Under
the assumption that the frame from our canonical example is selected
@code{(frame-first-window)} returns @code{W2}.
@end defun

@defun window-tree &optional frame
This function returns a list representing the window tree for frame
@var{frame}.  If @var{frame} is omitted or @code{nil}, it defaults to
the selected frame.

The return value is a list of the form @code{(@var{root} @var{mini})},
where @var{root} represents the window tree of the frame's root
window, and @var{mini} is the frame's minibuffer window.

If the root window is live, @var{root} is that window itself.
Otherwise, @var{root} is a list @code{(@var{dir} @var{edges} @var{w1}
@var{w2} ...)} where @var{dir} is @code{nil} for a horizontal
combination and @code{t} for a vertical combination, @var{edges} gives
the size and position of the combination, and the remaining elements
are the child windows.  Each child window may again be a window object
(for a live window) or a list with the same format as above (for an
internal window).  The @var{edges} element is a list @code{(@var{left}
@var{top} @var{right} @var{bottom})}, similar to the value returned by
@code{window-edges} (@pxref{Coordinates and Windows}).
@end defun

@node Window Sizes
@section Window Sizes
@cindex window size
@cindex size of window

  The following schematic shows the structure of a live window:

@smallexample
@group
         _________________________________________
      ^ |______________ Header Line_______________|
      | |LS|LF|LM|                       |RM|RF|RS| ^
      | |  |  |  |                       |  |  |  | |
 Window |  |  |  |       Text Area       |  |  |  | Window
 Total  |  |  |  |     (Window Body)     |  |  |  | Body
 Height |  |  |  |                       |  |  |  | Height
      | |  |  |  |<- Window Body Width ->|  |  |  | |
      | |__|__|__|_______________________|__|__|__| v
      v |_______________ Mode Line _______________|

         <----------- Window Total Width -------->

@end group
@end smallexample

@cindex window body
@cindex text area of a window
@cindex body of a window
  At the center of the window is the @dfn{text area}, or @dfn{body},
where the buffer text is displayed.  On each side of the text area is
a series of vertical areas; from innermost to outermost, these are the
left and right margins, denoted by LM and RM in the schematic
(@pxref{Display Margins}); the left and right fringes, denoted by LF
and RF (@pxref{Fringes}); and the left or right scroll bar, only one of
which is present at any time, denoted by LS and RS (@pxref{Scroll
Bars}).  At the top of the window is an optional header line
(@pxref{Header Lines}), and at the bottom of the window is the mode
line (@pxref{Mode Line Format}).

  Emacs provides several functions for finding the height and width of
a window.  Except where noted, Emacs reports window heights and widths
as integer numbers of lines and columns, respectively.  On a graphical
display, each ``line'' and ``column'' actually corresponds to the
height and width of a ``default'' character specified by the frame's
default font.  Thus, if a window is displaying text with a different
font or size, the reported height and width for that window may differ
from the actual number of text lines or columns displayed within it.

@cindex window height
@cindex height of a window
@cindex total height of a window
@cindex window width
@cindex width of a window
@cindex total width of a window
  The @dfn{total height} of a window is the distance between the top
and bottom of the window, including the header line (if one exists)
and the mode line.  The @dfn{total width} of a window is the distance
between the left and right edges of the mode line.  Note that the
height of a frame is not the same as the height of its windows, since
a frame may also contain an echo area, menu bar, and tool bar
(@pxref{Size and Position}).

@defun window-total-height &optional window
This function returns the total height, in lines, of the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window.  If @var{window} is an internal window, the
return value is the total height occupied by its descendant windows.
@end defun

@defun window-total-width &optional window
This function returns the total width, in columns, of the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window.  If @var{window} is internal, the return value
is the total width occupied by its descendant windows.
@end defun

@defun window-total-size &optional window horizontal
This function returns either the total height or width of the window
@var{window}.  If @var{horizontal} is omitted or @code{nil}, this is
equivalent to calling @code{window-total-height} for @var{window};
otherwise it is equivalent to calling @code{window-total-width} for
@var{window}.
@end defun

@cindex full-width window
@cindex full-height window
  The following functions can be used to determine whether a given
window has any adjacent windows.

@defun window-full-height-p &optional window
This function returns non-@code{nil} if @var{window} has no other
window above or below it in its frame, i.e. its total height equals
the total height of the root window on that frame.  If @var{window} is
omitted or @code{nil}, it defaults to the selected window.
@end defun

@defun window-full-width-p &optional window
This function returns non-@code{nil} if @var{window} has no other
window to the left or right in its frame, i.e. its total width equals
that of the root window on that frame.  If @var{window} is omitted or
@code{nil}, it defaults to the selected window.
@end defun

@cindex window body height
@cindex body height of a window
@cindex window body width
@cindex body width of a window
@cindex body size of a window
@cindex window body size
  The @dfn{body height} of a window is the height of its text area,
which does not include the mode or header line.  Similarly, the
@dfn{body width} is the width of the text area, which does not include
the scroll bar, fringes, or margins.

@defun window-body-height &optional window
This function returns the body height, in lines, of the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window; otherwise it must be a live window.

If there is a partially-visible line at the bottom of the text area,
that counts as a whole line; to exclude such a partially-visible line,
use @code{window-text-height}, below.
@end defun

@defun window-body-width &optional window
This function returns the body width, in columns, of the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window; otherwise it must be a live window.
@end defun

@defun window-body-size &optional window horizontal
This function returns the body height or body width of @var{window}.
If @var{horizontal} is omitted or @code{nil}, it is equivalent to
calling @code{window-body-height} for @var{window}; otherwise it is
equivalent to calling @code{window-body-width}.
@end defun

@defun window-text-height &optional window
This function is like @code{window-body-height}, except that any
partially-visible line at the bottom of the text area is not counted.
@end defun

  For compatibility with previous versions of Emacs,
@code{window-height} is an alias for @code{window-total-height}, and
@code{window-width} is an alias for @code{window-body-width}.  These
aliases are considered obsolete and will be removed in the future.

@cindex fixed-size window
  Commands that change the size of windows (@pxref{Resizing Windows}),
or split them (@pxref{Splitting Windows}), obey the variables
@code{window-min-height} and @code{window-min-width}, which specify
the smallest allowable window height and width.  @xref{Change
Window,,Deleting and Rearranging Windows, emacs, The GNU Emacs
Manual}.  They also obey the variable @code{window-size-fixed}, with
which a window can be @dfn{fixed} in size:

@defvar window-size-fixed
If this buffer-local variable is non-@code{nil}, the size of any
window displaying the buffer cannot normally be changed.  Deleting a
window or changing the frame's size may still change its size, if
there is no choice.

If the value is @code{height}, then only the window's height is fixed;
if the value is @code{width}, then only the window's width is fixed.
Any other non-@code{nil} value fixes both the width and the height.
@end defvar

@defun window-size-fixed-p &optional window horizontal
This function returns a non-@code{nil} value if @var{window}'s height
is fixed.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window.  If the optional argument @var{horizontal} is
non-@code{nil}, the return value is non-@code{nil} if @var{window}'s
width is fixed.

A @code{nil} return value does not necessarily mean that @var{window}
can be resized in the desired direction.  To determine that, use the
function @code{window-resizable}.  @xref{Resizing Windows}.
@end defun

  @xref{Coordinates and Windows}, for more functions that report the
positions of various parts of a window relative to the frame, from
which you can calculate its size.  In particular, you can use the
functions @code{window-pixel-edges} and
@code{window-inside-pixel-edges} to find the size in pixels, for
graphical displays.

@node Resizing Windows
@section Resizing Windows
@cindex window resizing
@cindex resize window
@cindex changing window size
@cindex window size, changing

  This section describes functions for resizing a window without
changing the size of its frame.  Because live windows do not overlap,
these functions are meaningful only on frames that contain two or more
windows: resizing a window also changes the size of a neighboring
window.  If there is just one window on a frame, its size cannot be
changed except by resizing the frame (@pxref{Size and Position}).

  Except where noted, these functions also accept internal windows as
arguments.  Resizing an internal window causes its child windows to be
resized to fit the same space.

@defun window-resizable window delta &optional horizontal ignore
This function returns @var{delta} if the size of @var{window} can be
changed vertically by @var{delta} lines.  If the optional argument
@var{horizontal} is non-@code{nil}, it instead returns @var{delta} if
@var{window} can be resized horizontally by @var{delta} columns.  It
does not actually change the window size.

If @var{window} is @code{nil}, it defaults to the selected window.

A positive value of @var{delta} means to check whether the window can be
enlarged by that number of lines or columns; a negative value of
@var{delta} means to check whether the window can be shrunk by that many
lines or columns.  If @var{delta} is non-zero, a return value of 0 means
that the window cannot be resized.

Normally, the variables @code{window-min-height} and
@code{window-min-width} specify the smallest allowable window size.
@xref{Change Window,, Deleting and Rearranging Windows, emacs, The GNU
Emacs Manual}.  However, if the optional argument @var{ignore} is
non-@code{nil}, this function ignores @code{window-min-height} and
@code{window-min-width}, as well as @code{window-size-fixed}.
Instead, it considers the minimum-height window to be one consisting
of a header (if any), a mode line, plus a text area one line tall; and
a minimum-width window as one consisting of fringes, margins, and
scroll bar (if any), plus a text area two columns wide.
@end defun

@defun window-resize window delta &optional horizontal ignore
This function resizes @var{window} by @var{delta} increments.  If
@var{horizontal} is @code{nil}, it changes the height by @var{delta}
lines; otherwise, it changes the width by @var{delta} columns.  A
positive @var{delta} means to enlarge the window, and a negative
@var{delta} means to shrink it.

If @var{window} is @code{nil}, it defaults to the selected window.  If
the window cannot be resized as demanded, an error is signaled.

The optional argument @var{ignore} has the same meaning as for the
function @code{window-resizable} above.

The choice of which window edges this function alters depends on the
values of the option @code{window-combination-resize} and the
combination limits of the involved windows; in some cases, it may alter
both edges.  @xref{Recombining Windows}.  To resize by moving only the
bottom or right edge of a window, use the function
@code{adjust-window-trailing-edge}, below.
@end defun

@c The commands enlarge-window, enlarge-window-horizontally,
@c shrink-window, and shrink-window-horizontally are documented in the
@c Emacs manual.  They are not preferred for calling from Lisp.

@defun adjust-window-trailing-edge window delta &optional horizontal
This function moves @var{window}'s bottom edge by @var{delta} lines.
If optional argument @var{horizontal} is non-@code{nil}, it instead
moves the right edge by @var{delta} columns.  If @var{window} is
@code{nil}, it defaults to the selected window.

A positive @var{delta} moves the edge downwards or to the right; a
negative @var{delta} moves it upwards or to the left.  If the edge
cannot be moved as far as specified by @var{delta}, this function
moves it as far as possible but does not signal a error.

This function tries to resize windows adjacent to the edge that is
moved.  If this is not possible for some reason (e.g. if that adjacent
window is fixed-size), it may resize other windows.
@end defun

  The following commands resize windows in more specific ways.  When
called interactively, they act on the selected window.

@deffn Command fit-window-to-buffer &optional window max-height min-height override
This command adjusts the height of @var{window} to fit the text in it.
It returns non-@code{nil} if it was able to resize @var{window}, and
@code{nil} otherwise.  If @var{window} is omitted or @code{nil}, it
defaults to the selected window.  Otherwise, it should be a live
window.

The optional argument @var{max-height}, if non-@code{nil}, specifies
the maximum total height that this function can give @var{window}.
The optional argument @var{min-height}, if non-@code{nil}, specifies
the minimum total height that it can give, which overrides the
variable @code{window-min-height}.

If the optional argument @var{override} is non-@code{nil}, this
function ignores any size restrictions imposed by
@code{window-min-height} and @code{window-min-width}.
@end deffn

@deffn Command shrink-window-if-larger-than-buffer &optional window
This command attempts to reduce @var{window}'s height as much as
possible while still showing its full buffer, but no less than
@code{window-min-height} lines.  The return value is non-@code{nil} if
the window was resized, and @code{nil} otherwise.  If @var{window} is
omitted or @code{nil}, it defaults to the selected window.  Otherwise,
it should be a live window.

This command does nothing if the window is already too short to
display all of its buffer, or if any of the buffer is scrolled
off-screen, or if the window is the only live window in its frame.
@end deffn

@cindex balancing window sizes
@deffn Command balance-windows &optional window-or-frame
This function balances windows in a way that gives more space to
full-width and/or full-height windows.  If @var{window-or-frame}
specifies a frame, it balances all windows on that frame.  If
@var{window-or-frame} specifies a window, it balances only that window
and its siblings (@pxref{Windows and Frames}).
@end deffn

@deffn Command balance-windows-area
This function attempts to give all windows on the selected frame
approximately the same share of the screen area.  Full-width or
full-height windows are not given more space than other windows.
@end deffn

@cindex maximizing windows
@deffn Command maximize-window &optional window
This function attempts to make @var{window} as large as possible, in
both dimensions, without resizing its frame or deleting other windows.
If @var{window} is omitted or @code{nil}, it defaults to the selected
window.
@end deffn

@cindex minimizing windows
@deffn Command minimize-window &optional window
This function attempts to make @var{window} as small as possible, in
both dimensions, without deleting it or resizing its frame.  If
@var{window} is omitted or @code{nil}, it defaults to the selected
window.
@end deffn


@node Splitting Windows
@section Splitting Windows
@cindex splitting windows
@cindex window splitting

This section describes functions for creating a new window by
@dfn{splitting} an existing one.

@deffn Command split-window &optional window size side
This function creates a new live window next to the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window.  That window is ``split'', and reduced in
size.  The space is taken up by the new window, which is returned.

The optional second argument @var{size} determines the sizes of
@var{window} and/or the new window.  If it is omitted or @code{nil},
both windows are given equal sizes; if there is an odd line, it is
allocated to the new window.  If @var{size} is a positive number,
@var{window} is given @var{size} lines (or columns, depending on the
value of @var{side}).  If @var{size} is a negative number, the new
window is given @minus{}@var{size} lines (or columns).

If @var{size} is @code{nil}, this function obeys the variables
@code{window-min-height} and @code{window-min-width}.  @xref{Change
Window,,Deleting and Rearranging Windows, emacs, The GNU Emacs
Manual}.  Thus, it signals an error if splitting would result in
making a window smaller than those variables specify.  However, a
non-@code{nil} value for @var{size} causes those variables to be
ignored; in that case, the smallest allowable window is considered to
be one that has space for a text area one line tall and/or two columns
wide.

The optional third argument @var{side} determines the position of the
new window relative to @var{window}.  If it is @code{nil} or
@code{below}, the new window is placed below @var{window}.  If it is
@code{above}, the new window is placed above @var{window}.  In both
these cases, @var{size} specifies a total window height, in lines.

If @var{side} is @code{t} or @code{right}, the new window is placed on
the right of @var{window}.  If @var{side} is @code{left}, the new
window is placed on the left of @var{window}.  In both these cases,
@var{size} specifies a total window width, in columns.

If @var{window} is a live window, the new window inherits various
properties from it, including margins and scroll bars.  If
@var{window} is an internal window, the new window inherits the
properties of the window selected within @var{window}'s frame.

The behavior of this function may be altered by the window parameters
of @var{window}, so long as the variable
@code{ignore-window-parameters} is @code{nil}.  If the value of
the @code{split-window} window parameter is @code{t}, this function
ignores all other window parameters.  Otherwise, if the value of the
@code{split-window} window parameter is a function, that function is
called with the arguments @var{window}, @var{size}, and @var{side}, in
lieu of the usual action of @code{split-window}.  Otherwise, this
function obeys the @code{window-atom} or @code{window-side} window
parameter, if any.  @xref{Window Parameters}.
@end deffn

  As an example, here is a sequence of @code{split-window} calls that
yields the window configuration discussed in @ref{Windows and Frames}.
This example demonstrates splitting a live window as well as splitting
an internal window.  We begin with a frame containing a single window
(a live root window), which we denote by @var{W4}.  Calling
@code{(split-window W4)} yields this window configuration:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W5_________________||
    |__________________W3__________________|

@end group
@end smallexample

@noindent
The @code{split-window} call has created a new live window, denoted by
@var{W5}.  It has also created a new internal window, denoted by
@var{W3}, which becomes the root window and the parent of both
@var{W4} and @var{W5}.

  Next, we call @code{(split-window W3 nil 'left)}, passing the
internal window @var{W3} as the argument.  The result:

@smallexample
@group
     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|
@end group
@end smallexample

@noindent
A new live window @var{W2} is created, to the left of the internal
window @var{W3}.  A new internal window @var{W1} is created, becoming
the new root window.

   For interactive use, Emacs provides two commands which always split
the selected window.  These call @code{split-window} internally.

@deffn Command split-window-right &optional size
This function splits the selected window into two side-by-side
windows, putting the selected window on the left.  If @var{size} is
positive, the left window gets @var{size} columns; if @var{size} is
negative, the right window gets @minus{}@var{size} columns.
@end deffn

@deffn Command split-window-below &optional size
This function splits the selected window into two windows, one above
the other, leaving the upper window selected.  If @var{size} is
positive, the upper window gets @var{size} lines; if @var{size} is
negative, the lower window gets @minus{}@var{size} lines.
@end deffn

@defopt split-window-keep-point
If the value of this variable is non-@code{nil} (the default),
@code{split-window-below} behaves as described above.

If it is @code{nil}, @code{split-window-below} adjusts point in each
of the two windows to minimize redisplay.  (This is useful on slow
terminals.)  It selects whichever window contains the screen line that
point was previously on.  Note that this only affects
@code{split-window-below}, not the lower-level @code{split-window}
function.
@end defopt

@node Deleting Windows
@section Deleting Windows
@cindex deleting windows

  @dfn{Deleting} a window removes it from the frame's window tree.  If
the window is a live window, it disappears from the screen.  If the
window is an internal window, its child windows are deleted too.

  Even after a window is deleted, it continues to exist as a Lisp
object, until there are no more references to it.  Window deletion can
be reversed, by restoring a saved window configuration (@pxref{Window
Configurations}).

@deffn Command delete-window &optional window
This function removes @var{window} from display and returns
@code{nil}.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window.  If deleting the window would leave no more
windows in the window tree (e.g. if it is the only live window in the
frame), an error is signaled.

By default, the space taken up by @var{window} is given to one of its
adjacent sibling windows, if any.  However, if the variable
@code{window-combination-resize} is non-@code{nil}, the space is
proportionally distributed among any remaining windows in the window
combination.  @xref{Recombining Windows}.

The behavior of this function may be altered by the window parameters
of @var{window}, so long as the variable
@code{ignore-window-parameters} is @code{nil}.  If the value of
the @code{delete-window} window parameter is @code{t}, this function
ignores all other window parameters.  Otherwise, if the value of the
@code{delete-window} window parameter is a function, that function is
called with the argument @var{window}, in lieu of the usual action of
@code{delete-window}.  Otherwise, this function obeys the
@code{window-atom} or @code{window-side} window parameter, if any.
@xref{Window Parameters}.
@end deffn

@deffn Command delete-other-windows &optional window
This function makes @var{window} fill its frame, by deleting other
windows as necessary.  If @var{window} is omitted or @code{nil}, it
defaults to the selected window.  The return value is @code{nil}.

The behavior of this function may be altered by the window parameters
of @var{window}, so long as the variable
@code{ignore-window-parameters} is @code{nil}.  If the value of
the @code{delete-other-windows} window parameter is @code{t}, this
function ignores all other window parameters.  Otherwise, if the value
of the @code{delete-other-windows} window parameter is a function,
that function is called with the argument @var{window}, in lieu of the
usual action of @code{delete-other-windows}.  Otherwise, this function
obeys the @code{window-atom} or @code{window-side} window parameter,
if any.  @xref{Window Parameters}.
@end deffn

@deffn Command delete-windows-on &optional buffer-or-name frame
This function deletes all windows showing @var{buffer-or-name}, by
calling @code{delete-window} on those windows.  @var{buffer-or-name}
should be a buffer, or the name of a buffer; if omitted or @code{nil},
it defaults to the current buffer.  If there are no windows showing
the specified buffer, this function does nothing.  If the specified
buffer is a minibuffer, an error is signaled.

If there is a dedicated window showing the buffer, and that window is
the only one on its frame, this function also deletes that frame if it
is not the only frame on the terminal.

The optional argument @var{frame} specifies which frames to operate
on:

@itemize @bullet
@item @code{nil}
means operate on all frames.
@item @code{t}
means operate on the selected frame.
@item @code{visible}
means operate on all visible frames.
@item @code{0}
means operate on all visible or iconified frames.
@item A frame
means operate on that frame.
@end itemize

Note that this argument does not have the same meaning as in other
functions which scan all live windows (@pxref{Cyclic Window
Ordering}).  Specifically, the meanings of @code{t} and @code{nil} here
are the opposite of what they are in those other functions.
@end deffn


@node Recombining Windows
@section Recombining Windows

When deleting the last sibling of a window @code{W}, its parent window
is deleted too, with @code{W} replacing it in the window tree.  This
means that @code{W} must be recombined with its parent's siblings to
form a new window combination (@pxref{Windows and Frames}).  In some
occasions, deleting a live window may even entail the deletion of two
internal windows.

@smallexample
@group
     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      ||| ___________  ___________ |||
    ||      ||||           ||           ||||
    ||      ||||____W6_____||_____W7____||||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Deleting @code{W5} in this configuration normally causes the deletion of
@code{W3} and @code{W4}.  The remaining live windows @code{W2},
@code{W6} and @code{W7} are recombined to form a new horizontal
combination with parent @code{W1}.

   Sometimes, however, it makes sense to not delete a parent window like
@code{W4}.  In particular, a parent window should not be removed when it
was used to preserve a combination embedded in a combination of the same
type.  Such embeddings make sense to assure that when you split a window
and subsequently delete the new window, Emacs reestablishes the layout
of the associated frame as it existed before the splitting.

   Consider a scenario starting with two live windows @code{W2} and
@code{W3} and their parent @code{W1}.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Split @code{W2} to make a new window @code{W4} as follows.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Now, when enlarging a window vertically, Emacs tries to obtain the
corresponding space from its lower sibling, provided such a window
exists.  In our scenario, enlarging @code{W4} will steal space from
@code{W3}.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Deleting @code{W4} will now give its entire space to @code{W2},
including the space earlier stolen from @code{W3}.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
This can be counterintutive, in particular if @code{W4} were used for
displaying a buffer only temporarily (@pxref{Temporary Displays}), and
you want to continue working with the initial layout.

The behavior can be fixed by making a new parent window when splitting
@code{W2}.  The variable described next allows to do that.

@defopt window-combination-limit
This variable controls whether splitting a window shall make a new
parent window.  The following values are recognized:

@table @code
@item nil
This means that the new live window is allowed to share the existing
parent window, if one exists, provided the split occurs in the same
direction as the existing window combination (otherwise, a new internal
window is created anyway).

@item window-size
In this case @code{display-buffer} makes a new parent window if it is
passed a @code{window-height} or @code{window-width} entry in the
@var{alist} argument (@pxref{Display Action Functions}).

@item temp-buffer
This value causes the creation of a new parent window when a window is
split for showing a temporary buffer (@pxref{Temporary Displays}) only.

@item display-buffer
This means that when @code{display-buffer} (@pxref{Choosing Window})
splits a window it always makes a new parent window.

@item t
In this case a new parent window is always created when splitting a
window.  Thus, if the value of this variable is at all times @code{t},
then at all times every window tree is a binary tree (a tree where each
window except the root window has exactly one sibling).
@end table

The default is @code{nil}.  Other values are reserved for future use.

If, as a consequence of this variable's setting, @code{split-window}
makes a new parent window, it also calls
@code{set-window-combination-limit} (see below) on the newly-created
internal window.  This affects how the window tree is rearranged when
the child windows are deleted (see below).
@end defopt

  If @code{window-combination-limit} is @code{t}, splitting @code{W2} in
the initial configuration of our scenario would have produced this:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    || __________________________________ ||
    |||                                  |||
    |||________________W2________________|||
    || __________________________________ ||
    |||                                  |||
    |||________________W4________________|||
    ||_________________W5_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
A new internal window @code{W5} has been created; its children are
@code{W2} and the new live window @code{W4}.  Now, @code{W2} is the only
sibling of @code{W4}, so enlarging @code{W4} will try to shrink
@code{W2}, leaving @code{W3} unaffected.  Observe that @code{W5}
represents a vertical combination of two windows embedded in the
vertical combination @code{W1}.

@cindex window combination limit
@defun set-window-combination-limit window limit
This functions sets the @dfn{combination limit} of the window
@var{window} to @var{limit}.  This value can be retrieved via the
function @code{window-combination-limit}.  See below for its effects;
note that it is only meaningful for internal windows.  The
@code{split-window} function automatically calls this function, passing
it @code{t} as @var{limit}, provided the value of the variable
@code{window-combination-limit} is @code{t} when it is called.
@end defun

@defun window-combination-limit window
This function returns the combination limit for @var{window}.

The combination limit is meaningful only for an internal window.  If it
is @code{nil}, then Emacs is allowed to automatically delete
@var{window}, in response to a window deletion, in order to group the
child windows of @var{window} with its sibling windows to form a new
window combination.  If the combination limit is @code{t}, the child
windows of @var{window} are never automatically recombined with its
siblings.

If, in the configuration shown at the beginning of this section, the
combination limit of @code{W4} (the parent window of @code{W6} and
@code{W7}) is @code{t}, deleting @code{W5} will not implicitly delete
@code{W4} too.
@end defun

Alternatively, the problems sketched above can be avoided by always
resizing all windows in the same combination whenever one of its windows
is split or deleted.  This also permits to split windows that would be
otherwise too small for such an operation.

@defopt window-combination-resize
If this variable is @code{nil}, @code{split-window} can only split a
window (denoted by @var{window}) if @var{window}'s screen area is large
enough to accommodate both itself and the new window.

If this variable is @code{t}, @code{split-window} tries to resize all
windows that are part of the same combination as @var{window}, in order
to accommodate the new window.  In particular, this may allow
@code{split-window} to succeed even if @var{window} is a fixed-size
window or too small to ordinarily split.  Furthermore, subsequently
resizing or deleting @var{window} may resize all other windows in its
combination.

The default is @code{nil}.  Other values are reserved for future use.
The value of this variable is ignored when
@code{window-combination-limit} is non-@code{nil}.
@end defopt

  To illustrate the effect of @code{window-combination-resize}, consider
the following frame layout.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
If @code{window-combination-resize} is @code{nil}, splitting window
@code{W3} leaves the size of @code{W2} unchanged:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||_________________W3_________________||
    | ____________________________________ |
    ||                                    ||
    ||_________________W4_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
If @code{window-combination-resize} is @code{t}, splitting @code{W3}
instead leaves all three live windows with approximately the same
height:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Deleting any of the live windows @code{W2}, @code{W3} or @code{W4} will
distribute its space proportionally among the two remaining live
windows.


@node Selecting Windows
@section Selecting Windows
@cindex selecting a window

@defun select-window window &optional norecord
This function makes @var{window} the selected window, as well as the
window selected within its frame (@pxref{Basic Windows}).  @var{window}
must be a live window.  This function makes also @var{window}'s buffer
current (@pxref{Buffers and Windows}).  The return value is
@var{window}.

By default, this function also moves @var{window}'s buffer to the front
of the buffer list (@pxref{The Buffer List}), and makes @var{window} the
most recently selected window.  However, if the optional argument
@var{norecord} is non-@code{nil}, these additional actions are omitted.
@end defun

@cindex most recently selected windows
  The sequence of calls to @code{select-window} with a non-@code{nil}
@var{norecord} argument determines an ordering of windows by their
selection time.  The function @code{get-lru-window} can be used to
retrieve the least recently selected live window (@pxref{Cyclic Window
Ordering}).

@defmac save-selected-window forms@dots{}
This macro records the selected frame, as well as the selected window
of each frame, executes @var{forms} in sequence, then restores the
earlier selected frame and windows.  It also saves and restores the
current buffer.  It returns the value of the last form in @var{forms}.

This macro does not save or restore anything about the sizes,
arrangement or contents of windows; therefore, if @var{forms} change
them, the change persists.  If the previously selected window of some
frame is no longer live at the time of exit from @var{forms}, that
frame's selected window is left alone.  If the previously selected
window is no longer live, then whatever window is selected at the end of
@var{forms} remains selected.  The current buffer is restored if and
only if it is still live when exiting @var{forms}.

This macro changes neither the ordering of recently selected windows nor
the buffer list.
@end defmac

@defmac with-selected-window window forms@dots{}
This macro selects @var{window}, executes @var{forms} in sequence, then
restores the previously selected window and current buffer.  The ordering
of recently selected windows and the buffer list remain unchanged unless
you deliberately change them within @var{forms}; for example, by calling
@code{select-window} with argument @var{norecord} @code{nil}.

This macro does not change the order of recently selected windows or
the buffer list.
@end defmac

@defun frame-selected-window &optional frame
This function returns the window on @var{frame} that is selected
within that frame.  @var{frame} should be a live frame; if omitted or
@code{nil}, it defaults to the selected frame.
@end defun

@defun set-frame-selected-window frame window &optional norecord
This function makes @var{window} the window selected within the frame
@var{frame}.  @var{frame} should be a live frame; if omitted or
@code{nil}, it defaults to the selected frame.  @var{window} should be
a live window; if omitted or @code{nil}, it defaults to the selected
window.

If @var{frame} is the selected frame, this makes @var{window} the
selected window.

If the optional argument @var{norecord} is non-@code{nil}, this
function does not alter the list of most recently selected windows,
nor the buffer list.
@end defun

@node Cyclic Window Ordering
@section Cyclic Ordering of Windows
@cindex cyclic ordering of windows
@cindex ordering of windows, cyclic
@cindex window ordering, cyclic

  When you use the command @kbd{C-x o} (@code{other-window}) to select
some other window, it moves through live windows in a specific order.
For any given configuration of windows, this order never varies.  It
is called the @dfn{cyclic ordering of windows}.

  The ordering is determined by a depth-first traversal of the frame's
window tree, retrieving the live windows which are the leaf nodes of
the tree (@pxref{Windows and Frames}).  If the minibuffer is active,
the minibuffer window is included too.  The ordering is cyclic, so the
last window in the sequence is followed by the first one.

@defun next-window &optional window minibuf all-frames
@cindex minibuffer window, and @code{next-window}
This function returns a live window, the one following @var{window} in
the cyclic ordering of windows.  @var{window} should be a live window;
if omitted or @code{nil}, it defaults to the selected window.

The optional argument @var{minibuf} specifies whether minibuffer windows
should be included in the cyclic ordering.  Normally, when @var{minibuf}
is @code{nil}, a minibuffer window is included only if it is currently
``active''; this matches the behavior of @kbd{C-x o}.  (Note that a
minibuffer window is active as long as its minibuffer is in use; see
@ref{Minibuffers}).

If @var{minibuf} is @code{t}, the cyclic ordering includes all
minibuffer windows.  If @var{minibuf} is neither @code{t} nor
@code{nil}, minibuffer windows are not included even if they are active.

The optional argument @var{all-frames} specifies which frames to
consider:

@itemize @bullet
@item @code{nil}
means to consider windows on @var{window}'s frame.  If the minibuffer
window is considered (as specified by the @var{minibuf} argument),
then frames that share the minibuffer window are considered too.

@item @code{t}
means to consider windows on all existing frames.

@item @code{visible}
means to consider windows on all visible frames.

@item 0
means to consider windows on all visible or iconified frames.

@item A frame
means to consider windows on that specific frame.

@item Anything else
means to consider windows on @var{window}'s frame, and no others.
@end itemize

If more than one frame is considered, the cyclic ordering is obtained
by appending the orderings for those frames, in the same order as the
list of all live frames (@pxref{Finding All Frames}).
@end defun

@defun previous-window &optional window minibuf all-frames
This function returns a live window, the one preceding @var{window} in
the cyclic ordering of windows.  The other arguments are handled like
in @code{next-window}.
@end defun

@deffn Command other-window count &optional all-frames
This function selects a live window, one @var{count} places from the
selected window in the cyclic ordering of windows.  If @var{count} is
a positive number, it skips @var{count} windows forwards; if
@var{count} is negative, it skips @minus{}@var{count} windows
backwards; if @var{count} is zero, that simply re-selects the selected
window.  When called interactively, @var{count} is the numeric prefix
argument.

The optional argument @var{all-frames} has the same meaning as in
@code{next-window}, like a @code{nil} @var{minibuf} argument to
@code{next-window}.

This function does not select a window that has a non-@code{nil}
@code{no-other-window} window parameter (@pxref{Window Parameters}).
@end deffn

@defun walk-windows fun &optional minibuf all-frames
This function calls the function @var{fun} once for each live window,
with the window as the argument.

It follows the cyclic ordering of windows.  The optional arguments
@var{minibuf} and @var{all-frames} specify the set of windows
included; these have the same arguments as in @code{next-window}.  If
@var{all-frames} specifies a frame, the first window walked is the
first window on that frame (the one returned by
@code{frame-first-window}), not necessarily the selected window.

If @var{fun} changes the window configuration by splitting or deleting
windows, that does not alter the set of windows walked, which is
determined prior to calling @var{fun} for the first time.
@end defun

@defun one-window-p &optional no-mini all-frames
This function returns @code{t} if the selected window is the only live
window, and @code{nil} otherwise.

If the minibuffer window is active, it is normally considered (so that
this function returns @code{nil}).  However, if the optional argument
@var{no-mini} is non-@code{nil}, the minibuffer window is ignored even
if active.  The optional argument @var{all-frames} has the same
meaning as for @code{next-window}.
@end defun

@cindex finding windows
  The following functions return a window which satisfies some
criterion, without selecting it:

@cindex least recently used window
@defun get-lru-window &optional all-frames dedicated not-selected
This function returns a live window which is heuristically the ``least
recently used'' window.  The optional argument @var{all-frames} has
the same meaning as in @code{next-window}.

If any full-width windows are present, only those windows are
considered.  A minibuffer window is never a candidate.  A dedicated
window (@pxref{Dedicated Windows}) is never a candidate unless the
optional argument @var{dedicated} is non-@code{nil}.  The selected
window is never returned, unless it is the only candidate.  However, if
the optional argument @var{not-selected} is non-@code{nil}, this
function returns @code{nil} in that case.
@end defun

@cindex largest window
@defun get-largest-window &optional all-frames dedicated not-selected
This function returns the window with the largest area (height times
width).  The optional argument @var{all-frames} specifies the windows to
search, and has the same meaning as in @code{next-window}.

A minibuffer window is never a candidate.  A dedicated window
(@pxref{Dedicated Windows}) is never a candidate unless the optional
argument @var{dedicated} is non-@code{nil}.  The selected window is not
a candidate if the optional argument @var{not-selected} is
non-@code{nil}.  If the optional argument @var{not-selected} is
non-@code{nil} and the selected window is the only candidate, this
function returns @code{nil}.

If there are two candidate windows of the same size, this function
prefers the one that comes first in the cyclic ordering of windows,
starting from the selected window.
@end defun

@cindex window that satisfies a predicate
@cindex conditional selection of windows
@defun get-window-with-predicate predicate &optional minibuf all-frames default
This function calls the function @var{predicate} for each of the
windows in the cyclic order of windows in turn, passing it the window
as an argument.  If the predicate returns non-@code{nil} for any
window, this function stops and returns that window.  If no such
window is found, the return value is @var{default} (which defaults to
@code{nil}).

The optional arguments @var{minibuf} and @var{all-frames} specify the
windows to search, and have the same meanings as in
@code{next-window}.
@end defun

@cindex window in direction
@defun window-in-direction direction &optional window ignore
This function returns the nearest window in direction @var{direction} as
seen from the position of @code{window-point} in window @var{window}.
The argument @var{direction} must be one of @code{above}, @code{below},
@code{left} or @code{right}.  The optional argument @var{window} must
denote a live window and defaults to the selected one.

This function does not return a window whose @code{no-other-window}
parameter is non-@code{nil}.  If the nearest window's
@code{no-other-window} parameter is non-@code{nil}, this function tries
to find another window in the indicated direction whose
@code{no-other-window} parameter is @code{nil}.  If the optional
argument @var{ignore} is non-@code{nil}, a window may be returned even
if its @code{no-other-window} parameter is non-@code{nil}.

If it doesn't find a suitable window, this function returns @code{nil}.
@end defun


@node Buffers and Windows
@section Buffers and Windows
@cindex examining windows
@cindex windows, controlling precisely
@cindex buffers, controlled in windows

  This section describes low-level functions for examining and setting
the contents of windows.  @xref{Switching Buffers}, for higher-level
functions for displaying a specific buffer in a window.

@defun window-buffer &optional window
This function returns the buffer that @var{window} is displaying.  If
@var{window} is omitted or @code{nil} it defaults to the selected
window.  If @var{window} is an internal window, this function returns
@code{nil}.
@end defun

@defun set-window-buffer window buffer-or-name &optional keep-margins
This function makes @var{window} display @var{buffer-or-name}.
@var{window} should be a live window; if @code{nil}, it defaults to
the selected window.  @var{buffer-or-name} should be a buffer, or the
name of an existing buffer.  This function does not change which
window is selected, nor does it directly change which buffer is
current (@pxref{Current Buffer}).  Its return value is @code{nil}.

If @var{window} is @dfn{strongly dedicated} to a buffer and
@var{buffer-or-name} does not specify that buffer, this function
signals an error.  @xref{Dedicated Windows}.

By default, this function resets @var{window}'s position, display
margins, fringe widths, and scroll bar settings, based on the local
variables in the specified buffer.  However, if the optional argument
@var{keep-margins} is non-@code{nil}, it leaves the display margins
and fringe widths unchanged.

When writing an application, you should normally use the higher-level
functions described in @ref{Switching Buffers}, instead of calling
@code{set-window-buffer} directly.

This runs @code{window-scroll-functions}, followed by
@code{window-configuration-change-hook}.  @xref{Window Hooks}.
@end defun

@defvar buffer-display-count
This buffer-local variable records the number of times a buffer has been
displayed in a window.  It is incremented each time
@code{set-window-buffer} is called for the buffer.
@end defvar

@defvar buffer-display-time
This buffer-local variable records the time at which a buffer was last
displayed in a window.  The value is @code{nil} if the buffer has
never been displayed.  It is updated each time
@code{set-window-buffer} is called for the buffer, with the value
returned by @code{current-time} (@pxref{Time of Day}).
@end defvar

@defun get-buffer-window &optional buffer-or-name all-frames
This function returns the first window displaying @var{buffer-or-name}
in the cyclic ordering of windows, starting from the selected window
(@pxref{Cyclic Window Ordering}).  If no such window exists, the
return value is @code{nil}.

@var{buffer-or-name} should be a buffer or the name of a buffer; if
omitted or @code{nil}, it defaults to the current buffer.  The
optional argument @var{all-frames} specifies which windows to
consider:

@itemize @bullet
@item
@code{t} means consider windows on all existing frames.
@item
@code{visible} means consider windows on all visible frames.
@item
0 means consider windows on all visible or iconified frames.
@item
A frame means consider windows on that frame only.
@item
Any other value means consider windows on the selected frame.
@end itemize

Note that these meanings differ slightly from those of the
@var{all-frames} argument to @code{next-window} (@pxref{Cyclic Window
Ordering}).  This function may be changed in a future version of Emacs
to eliminate this discrepancy.
@end defun

@defun get-buffer-window-list &optional buffer-or-name minibuf all-frames
This function returns a list of all windows currently displaying
@var{buffer-or-name}.  @var{buffer-or-name} should be a buffer or the
name of an existing buffer.  If omitted or @code{nil}, it defaults to
the current buffer.

The arguments @var{minibuf} and @var{all-frames} have the same
meanings as in the function @code{next-window} (@pxref{Cyclic Window
Ordering}).  Note that the @var{all-frames} argument does @emph{not}
behave exactly like in @code{get-buffer-window}.
@end defun

@deffn Command replace-buffer-in-windows &optional buffer-or-name
This command replaces @var{buffer-or-name} with some other buffer, in
all windows displaying it.  @var{buffer-or-name} should be a buffer,
or the name of an existing buffer; if omitted or @code{nil}, it
defaults to the current buffer.

The replacement buffer in each window is chosen via
@code{switch-to-prev-buffer} (@pxref{Window History}).  Any dedicated
window displaying @var{buffer-or-name} is deleted (@pxref{Dedicated
Windows}), unless it is the only window on its frame---if it is the
only window, and that frame is not the only frame on its terminal, the
frame is ``dismissed'' by calling the function specified by
@code{frame-auto-hide-function} (@pxref{Quitting Windows}).  If the
dedicated window is the only window on the only frame on its terminal,
the buffer is replaced anyway.
@end deffn

@node Switching Buffers
@section Switching to a Buffer in a Window
@cindex switching to a buffer
@cindex displaying a buffer

  This section describes high-level functions for switching to a
specified buffer in some window.

  Do @emph{not} use these functions to make a buffer temporarily
current just so a Lisp program can access or modify it.  They have
side-effects, such as changing window histories (@pxref{Window
History}), which will surprise the user if used that way.  If you want
to make a buffer current to modify it in Lisp, use
@code{with-current-buffer}, @code{save-current-buffer}, or
@code{set-buffer}.  @xref{Current Buffer}.

@deffn Command switch-to-buffer buffer-or-name &optional norecord force-same-window
This command attempts to display @var{buffer-or-name} in the selected
window, and makes it the current buffer.  It is often used
interactively (as the binding of @kbd{C-x b}), as well as in Lisp
programs.  The return value is the buffer switched to.

If @var{buffer-or-name} is @code{nil}, it defaults to the buffer
returned by @code{other-buffer} (@pxref{The Buffer List}).  If
@var{buffer-or-name} is a string that is not the name of any existing
buffer, this function creates a new buffer with that name; the new
buffer's major mode is determined by the variable @code{major-mode}
(@pxref{Major Modes}).

Normally, the specified buffer is put at the front of the buffer
list---both the global buffer list and the selected frame's buffer
list (@pxref{The Buffer List}).  However, this is not done if the
optional argument @var{norecord} is non-@code{nil}.

Sometimes, @code{switch-to-buffer} may be unable to display the buffer
in the selected window.  This happens if the selected window is a
minibuffer window, or if the selected window is strongly dedicated to
its buffer (@pxref{Dedicated Windows}).  In that case, the command
normally tries to display the buffer in some other window, by invoking
@code{pop-to-buffer} (see below).  However, if the optional argument
@var{force-same-window} is non-@code{nil}, it signals an error
instead.
@end deffn

By default, @code{switch-to-buffer} sets @code{window-point} of the
window used to the buffer's position of @code{point}.  This behavior can
be tuned using the following option.

@defopt switch-to-buffer-preserve-window-point
If this variable is @code{nil}, @code{switch-to-buffer} displays the
buffer specified by @var{buffer-or-name} at the position of that
buffer's @code{point}.  If this variable is @code{already-displayed}, it
tries to display the buffer at its previous position in the selected
window, provided the buffer is currently displayed in some other window
on any visible or iconified frame.  If this variable is @code{t},
@code{switch-to-buffer} unconditionally tries to display the buffer at
its previous position in the selected window.

This variable is ignored if the buffer is already displayed in the
selected window or never appeared in it before, or if
@code{switch-to-buffer} calls @code{pop-to-buffer} to display the
buffer.
@end defopt

The next two functions are similar to @code{switch-to-buffer}, except
for the described features.

@deffn Command switch-to-buffer-other-window buffer-or-name &optional norecord
This function makes the buffer specified by @var{buffer-or-name}
current and displays it in some window other than the selected window.
It uses the function @code{pop-to-buffer} internally (see below).

If the selected window already displays the specified buffer, it
continues to do so, but another window is nonetheless found to display
it as well.

The @var{buffer-or-name} and @var{norecord} arguments have the same
meanings as in @code{switch-to-buffer}.
@end deffn

@deffn Command switch-to-buffer-other-frame buffer-or-name &optional norecord
This function makes the buffer specified by @var{buffer-or-name}
current and displays it, usually in a new frame.  It uses the function
@code{pop-to-buffer} (see below).

If the specified buffer is already displayed in another window, in any
frame on the current terminal, this switches to that window instead of
creating a new frame.  However, the selected window is never used for
this.

The @var{buffer-or-name} and @var{norecord} arguments have the same
meanings as in @code{switch-to-buffer}.
@end deffn

The above commands use the function @code{pop-to-buffer}, which
flexibly displays a buffer in some window and selects that window for
editing.  In turn, @code{pop-to-buffer} uses @code{display-buffer} for
displaying the buffer.  Hence, all the variables affecting
@code{display-buffer} will affect it as well.  @xref{Choosing Window},
for the documentation of @code{display-buffer}.

@deffn Command pop-to-buffer buffer-or-name &optional action norecord
This function makes @var{buffer-or-name} the current buffer and
displays it in some window, preferably not the window previously
selected.  It then selects the displaying window.  If that window is
on a different graphical frame, that frame is given input focus if
possible (@pxref{Input Focus}).  The return value is the buffer that
was switched to.

If @var{buffer-or-name} is @code{nil}, it defaults to the buffer
returned by @code{other-buffer} (@pxref{The Buffer List}).  If
@var{buffer-or-name} is a string that is not the name of any existing
buffer, this function creates a new buffer with that name; the new
buffer's major mode is determined by the variable @code{major-mode}
(@pxref{Major Modes}).

If @var{action} is non-@code{nil}, it should be a display action to
pass to @code{display-buffer} (@pxref{Choosing Window}).
Alternatively, a non-@code{nil}, non-list value means to pop to a
window other than the selected one---even if the buffer is already
displayed in the selected window.

Like @code{switch-to-buffer}, this function updates the buffer list
unless @var{norecord} is non-@code{nil}.
@end deffn

@node Choosing Window
@section Choosing a Window for Display

  The command @code{display-buffer} flexibly chooses a window for
display, and displays a specified buffer in that window.  It can be
called interactively, via the key binding @kbd{C-x 4 C-o}.  It is also
used as a subroutine by many functions and commands, including
@code{switch-to-buffer} and @code{pop-to-buffer} (@pxref{Switching
Buffers}).

@cindex display action
@cindex action function, for @code{display-buffer}
@cindex action alist, for @code{display-buffer}
  This command performs several complex steps to find a window to
display in.  These steps are described by means of @dfn{display
actions}, which have the form @code{(@var{function} . @var{alist})}.
Here, @var{function} is either a function or a list of functions,
which we refer to as @dfn{action functions}; @var{alist} is an
association list, which we refer to as @dfn{action alists}.

  An action function accepts two arguments: the buffer to display and
an action alist.  It attempts to display the buffer in some window,
picking or creating a window according to its own criteria.  If
successful, it returns the window; otherwise, it returns @code{nil}.
@xref{Display Action Functions}, for a list of predefined action
functions.

  @code{display-buffer} works by combining display actions from
several sources, and calling the action functions in turn, until one
of them manages to display the buffer and returns a non-@code{nil}
value.

@deffn Command display-buffer buffer-or-name &optional action frame
This command makes @var{buffer-or-name} appear in some window, without
selecting the window or making the buffer current.  The argument
@var{buffer-or-name} must be a buffer or the name of an existing
buffer.  The return value is the window chosen to display the buffer.

The optional argument @var{action}, if non-@code{nil}, should normally
be a display action (described above).  @code{display-buffer} builds a
list of action functions and an action alist, by consolidating display
actions from the following sources (in order):

@itemize
@item
The variable @code{display-buffer-overriding-action}.

@item
The user option @code{display-buffer-alist}.

@item
The @var{action} argument.

@item
The user option @code{display-buffer-base-action}.

@item
The constant @code{display-buffer-fallback-action}.
@end itemize

@noindent
Each action function is called in turn, passing the buffer as the
first argument and the combined action alist as the second argument,
until one of the functions returns non-@code{nil}.

The argument @var{action} can also have a non-@code{nil}, non-list
value.  This has the special meaning that the buffer should be
displayed in a window other than the selected one, even if the
selected window is already displaying it.  If called interactively
with a prefix argument, @var{action} is @code{t}.

The optional argument @var{frame}, if non-@code{nil}, specifies which
frames to check when deciding whether the buffer is already displayed.
It is equivalent to adding an element @code{(reusable-frames
. @var{frame})} to the action alist of @var{action}.  @xref{Display
Action Functions}.
@end deffn

@defvar display-buffer-overriding-action
The value of this variable should be a display action, which is
treated with the highest priority by @code{display-buffer}.  The
default value is empty, i.e. @code{(nil . nil)}.
@end defvar

@defopt display-buffer-alist
The value of this option is an alist mapping regular expressions to
display actions.  If the name of the buffer passed to
@code{display-buffer} matches a regular expression in this alist, then
@code{display-buffer} uses the corresponding display action.
@end defopt

@defopt display-buffer-base-action
The value of this option should be a display action.  This option can
be used to define a ``standard'' display action for calls to
@code{display-buffer}.
@end defopt

@defvr Constant display-buffer-fallback-action
This display action specifies the fallback behavior for
@code{display-buffer} if no other display actions are given.
@end defvr

@node Display Action Functions
@section Action Functions for @code{display-buffer}

The following basic action functions are defined in Emacs.  Each of
these functions takes two arguments: @var{buffer}, the buffer to
display, and @var{alist}, an action alist.  Each action function
returns the window if it succeeds, and @code{nil} if it fails.

@defun display-buffer-same-window buffer alist
This function tries to display @var{buffer} in the selected window.
It fails if the selected window is a minibuffer window or is dedicated
to another buffer (@pxref{Dedicated Windows}).  It also fails if
@var{alist} has a non-@code{nil} @code{inhibit-same-window} entry.
@end defun

@defun display-buffer-reuse-window buffer alist
This function tries to ``display'' @var{buffer} by finding a window
that is already displaying it.

If @var{alist} has a non-@code{nil} @code{inhibit-same-window} entry,
the selected window is not eligible for reuse.  If @var{alist}
contains a @code{reusable-frames} entry, its value determines which
frames to search for a reusable window:

@itemize @bullet
@item
@code{nil} means consider windows on the selected frame.
(Actually, the last non-minibuffer frame.)
@item
@code{t} means consider windows on all frames.
@item
@code{visible} means consider windows on all visible frames.
@item
0 means consider windows on all visible or iconified frames.
@item
A frame means consider windows on that frame only.
@end itemize

If @var{alist} contains no @code{reusable-frames} entry, this function
normally searches just the selected frame; however, if the variable
@code{pop-up-frames} is non-@code{nil}, it searches all frames on the
current terminal.  @xref{Choosing Window Options}.

If this function chooses a window on another frame, it makes that
frame visible and raises it if necessary.
@end defun

@defun display-buffer-pop-up-frame buffer alist
This function creates a new frame, and displays the buffer in that
frame's window.  It actually performs the frame creation by calling
the function specified in @code{pop-up-frame-function}
(@pxref{Choosing Window Options}).
@end defun

@defun display-buffer-pop-up-window buffer alist
This function tries to display @var{buffer} by splitting the largest
or least recently-used window (typically one on the selected frame).
It actually performs the split by calling the function specified in
@code{split-window-preferred-function} (@pxref{Choosing Window
Options}).

The size of the new window can be adjusted by supplying
@code{window-height} and @code{window-width} entries in @var{alist}.  To
adjust the window's height, use an entry whose @sc{car} is
@code{window-height} and whose @sc{cdr} is one of:

@itemize @bullet
@item
@code{nil} means to leave the height of the new window alone.

@item
A number specifies the desired height of the new window.  An integer
number specifies the number of lines of the window.  A floating point
number gives the fraction of the window's height with respect to the
height of the frame's root window.

@item
If the @sc{cdr} specifies a function, that function is called with one
argument - the new window.  The function is supposed to adjust the
height of the window; its return value is ignored.  Suitable functions
are @code{shrink-window-if-larger-than-buffer} and
@code{fit-window-to-buffer}, see @ref{Resizing Windows}.
@end itemize

To adjust the window's width, use an entry whose @sc{car} is
@code{window-width} and whose @sc{cdr} is one of:

@itemize @bullet
@item
@code{nil} means to leave the width of the new window alone.

@item
A number specifies the desired width of the new window.  An integer
number specifies the number of columns of the window.  A floating point
number gives the fraction of the window's width with respect to the
width of the frame's root window.

@item
If the @sc{cdr} specifies a function, that function is called with one
argument - the new window.  The function is supposed to adjust the width
of the window; its return value is ignored.
@end itemize

This function can fail if no window splitting can be performed for some
reason (e.g. if there is just one frame and it has an
@code{unsplittable} frame parameter; @pxref{Buffer Parameters}).
@end defun

@defun display-buffer-use-some-window buffer alist
This function tries to display @var{buffer} by choosing an existing
window and displaying the buffer in that window.  It can fail if all
windows are dedicated to another buffer (@pxref{Dedicated Windows}).
@end defun

@defun display-buffer-below-selected buffer alist
This function tries to display @var{buffer} in a window below the
selected window.  This means to either split the selected window or
reuse the window below the selected one.
@end defun

@defun display-buffer-in-previous-window buffer alist
This function tries to display @var{buffer} in a window previously
showing it.  If @var{alist} has a non-@code{nil}
@code{inhibit-same-window} entry, the selected window is not eligible
for reuse.  If @var{alist} contains a @code{reusable-frames} entry, its
value determines which frames to search for a suitable window as with
@code{display-buffer-reuse-window}.

If @var{alist} has a @code{previous-window} entry, the window
specified by that entry will override any other window found by the
methods above, even if that window never showed @var{buffer} before.
@end defun


@node Choosing Window Options
@section Additional Options for Displaying Buffers

The behavior of the standard display actions of @code{display-buffer}
(@pxref{Choosing Window}) can be modified by a variety of user
options.

@defopt pop-up-windows
If the value of this variable is non-@code{nil}, @code{display-buffer}
is allowed to split an existing window to make a new window for
displaying in.  This is the default.

This variable is provided mainly for backward compatibility.  It is
obeyed by @code{display-buffer} via a special mechanism in
@code{display-buffer-fallback-action}, which only calls the action
function @code{display-buffer-pop-up-window} (@pxref{Display Action
Functions}) when the value is @code{nil}.  It is not consulted by
@code{display-buffer-pop-up-window} itself, which the user may specify
directly in @code{display-buffer-alist} etc.
@end defopt

@defopt split-window-preferred-function
This variable specifies a function for splitting a window, in order to
make a new window for displaying a buffer.  It is used by the
@code{display-buffer-pop-up-window} action function to actually split
the window (@pxref{Display Action Functions}).

The default value is @code{split-window-sensibly}, which is documented
below.  The value must be a function that takes one argument, a
window, and return either a new window (which is used to display the
desired buffer) or @code{nil} (which means the splitting failed).
@end defopt

@defun split-window-sensibly window
This function tries to split @var{window}, and return the newly
created window.  If @var{window} cannot be split, it returns
@code{nil}.

This function obeys the usual rules that determine when a window may
be split (@pxref{Splitting Windows}).  It first tries to split by
placing the new window below, subject to the restriction imposed by
@code{split-height-threshold} (see below), in addition to any other
restrictions.  If that fails, it tries to split by placing the new
window to the right, subject to @code{split-width-threshold} (see
below).  If that fails, and the window is the only window on its
frame, this function again tries to split and place the new window
below, disregarding @code{split-height-threshold}.  If this fails as
well, this function gives up and returns @code{nil}.
@end defun

@defopt split-height-threshold
This variable, used by @code{split-window-sensibly}, specifies whether
to split the window placing the new window below.  If it is an
integer, that means to split only if the original window has at least
that many lines.  If it is @code{nil}, that means not to split this
way.
@end defopt

@defopt split-width-threshold
This variable, used by @code{split-window-sensibly}, specifies whether
to split the window placing the new window to the right.  If the value
is an integer, that means to split only if the original window has at
least that many columns.  If the value is @code{nil}, that means not
to split this way.
@end defopt

@defopt pop-up-frames
If the value of this variable is non-@code{nil}, that means
@code{display-buffer} may display buffers by making new frames.  The
default is @code{nil}.

A non-@code{nil} value also means that when @code{display-buffer} is
looking for a window already displaying @var{buffer-or-name}, it can
search any visible or iconified frame, not just the selected frame.

This variable is provided mainly for backward compatibility.  It is
obeyed by @code{display-buffer} via a special mechanism in
@code{display-buffer-fallback-action}, which calls the action function
@code{display-buffer-pop-up-frame} (@pxref{Display Action Functions})
if the value is non-@code{nil}.  (This is done before attempting to
split a window.)  This variable is not consulted by
@code{display-buffer-pop-up-frame} itself, which the user may specify
directly in @code{display-buffer-alist} etc.
@end defopt

@defopt pop-up-frame-function
This variable specifies a function for creating a new frame, in order
to make a new window for displaying a buffer.  It is used by the
@code{display-buffer-pop-up-frame} action function (@pxref{Display
Action Functions}).

The value should be a function that takes no arguments and returns a
frame, or @code{nil} if no frame could be created.  The default value
is a function that creates a frame using the parameters specified by
@code{pop-up-frame-alist} (see below).
@end defopt

@defopt pop-up-frame-alist
This variable holds an alist of frame parameters (@pxref{Frame
Parameters}), which is used by the default function in
@code{pop-up-frame-function} to make a new frame.  The default is
@code{nil}.
@end defopt

@defopt same-window-buffer-names
A list of buffer names for buffers that should be displayed in the
selected window.  If a buffer's name is in this list,
@code{display-buffer} handles the buffer by switching to it in the
selected window.
@end defopt

@defopt same-window-regexps
A list of regular expressions that specify buffers that should be
displayed in the selected window.  If the buffer's name matches any of
the regular expressions in this list, @code{display-buffer} handles the
buffer by switching to it in the selected window.
@end defopt

@defun same-window-p buffer-name
This function returns @code{t} if displaying a buffer
named @var{buffer-name} with @code{display-buffer} would
put it in the selected window.
@end defun

@node Window History
@section Window History
@cindex window history

Each window remembers the buffers it has previously displayed, and the order
in which these buffers were removed from it.  This history is used,
for example, by @code{replace-buffer-in-windows} (@pxref{Buffers and
Windows}).  This list is automatically maintained by Emacs, but you can
use the following functions to explicitly inspect or alter it:

@defun window-prev-buffers &optional window
This function returns a list specifying the previous contents of
@var{window}, which should be a live window and defaults to the
selected window.

Each list element has the form @code{(@var{buffer} @var{window-start}
@var{window-pos})}, where @var{buffer} is a buffer previously shown in
the window, @var{window-start} is the window start position when that
buffer was last shown, and @var{window-pos} is the point position when
that buffer was last shown.

The list is ordered so that earlier elements correspond to more
recently-shown buffers, and the first element usually corresponds to the
buffer most recently removed from the window.
@end defun

@defun set-window-prev-buffers window prev-buffers
This function sets @var{window}'s previous buffers to the value of
@var{prev-buffers}.  The argument @var{window} must be a live window
and defaults to the selected one.  The argument @var{prev-buffers}
should be a list of the same form as that returned by
@code{window-prev-buffers}.
@end defun

In addition, each buffer maintains a list of @dfn{next buffers}, which
is a list of buffers re-shown by @code{switch-to-prev-buffer} (see
below).  This list is mainly used by @code{switch-to-prev-buffer} and
@code{switch-to-next-buffer} for choosing buffers to switch to.

@defun window-next-buffers &optional window
This function returns the list of buffers recently re-shown in
@var{window} via @code{switch-to-prev-buffer}.  The @var{window}
argument must denote a live window or @code{nil} (meaning the selected
window).
@end defun

@defun set-window-next-buffers window next-buffers
This function sets the next buffer list of @var{window} to
@var{next-buffers}.  The @var{window} argument should be a live window
or @code{nil} (meaning the selected window).  The argument
@var{next-buffers} should be a list of buffers.
@end defun

The following commands can be used to cycle through the global buffer
list, much like @code{bury-buffer} and @code{unbury-buffer}.  However,
they cycle according to the specified window's history list, rather
than the global buffer list.  In addition, they restore
window-specific window start and point positions, and may show a
buffer even if it is already shown in another window.  The
@code{switch-to-prev-buffer} command, in particular, is used by
@code{replace-buffer-in-windows}, @code{bury-buffer} and
@code{quit-window} to find a replacement buffer for a window.

@deffn Command switch-to-prev-buffer &optional window bury-or-kill
This command displays the previous buffer in @var{window}.  The
argument @var{window} should be a live window or @code{nil} (meaning
the selected window).  If the optional argument @var{bury-or-kill} is
non-@code{nil}, this means that the buffer currently shown in
@var{window} is about to be buried or killed and consequently should
not be switched to in future invocations of this command.

The previous buffer is usually the buffer shown before the buffer
currently shown in @var{window}.  However, a buffer that has been buried
or killed, or has been already shown by a recent invocation of
@code{switch-to-prev-buffer}, does not qualify as previous buffer.

If repeated invocations of this command have already shown all buffers
previously shown in @var{window}, further invocations will show buffers
from the buffer list of the frame @var{window} appears on (@pxref{The
Buffer List}), trying to skip buffers that are already shown in another
window on that frame.
@end deffn

@deffn Command switch-to-next-buffer &optional window
This command switches to the next buffer in @var{window}, thus undoing
the effect of the last @code{switch-to-prev-buffer} command in
@var{window}.  The argument @var{window} must be a live window and
defaults to the selected one.

If there is no recent invocation of @code{switch-to-prev-buffer} that
can be undone, this function tries to show a buffer from the buffer list
of the frame @var{window} appears on (@pxref{The Buffer List}).
@end deffn

By default @code{switch-to-prev-buffer} and @code{switch-to-next-buffer}
can switch to a buffer that is already shown in another window on the
same frame.  The following option can be used to override this behavior.

@defopt switch-to-visible-buffer
If this variable is non-@code{nil}, @code{switch-to-prev-buffer} and
@code{switch-to-next-buffer} may switch to a buffer that is already
visible on the same frame, provided the buffer was shown in the relevant
window before.  If it is @code{nil}, @code{switch-to-prev-buffer} and
@code{switch-to-next-buffer} always try to avoid switching to a buffer
that is already visible in another window on the same frame.
@end defopt


@node Dedicated Windows
@section Dedicated Windows
@cindex dedicated window

Functions for displaying a buffer can be told to not use specific
windows by marking these windows as @dfn{dedicated} to their buffers.
@code{display-buffer} (@pxref{Choosing Window}) never uses a dedicated
window for displaying another buffer in it.  @code{get-lru-window} and
@code{get-largest-window} (@pxref{Selecting Windows}) do not consider
dedicated windows as candidates when their @var{dedicated} argument is
non-@code{nil}.  The behavior of @code{set-window-buffer}
(@pxref{Buffers and Windows}) with respect to dedicated windows is
slightly different, see below.

When @code{delete-windows-on} (@pxref{Deleting Windows}) wants to
delete a dedicated window and that window is the only window on its
frame, it deletes the window's frame too, provided there are other
frames left.  @code{replace-buffer-in-windows} (@pxref{Switching
Buffers}) tries to delete all dedicated windows showing its buffer
argument.  When such a window is the only window on its frame, that
frame is deleted, provided there are other frames left.  If there are
no more frames left, some other buffer is displayed in the window, and
the window is marked as non-dedicated.

When you kill a buffer (@pxref{Killing Buffers}) displayed in a
dedicated window, any such window usually gets deleted too, since
@code{kill-buffer} calls @code{replace-buffer-in-windows} for cleaning
up windows.  Burying a buffer (@pxref{The Buffer List}) deletes the
selected window if it is dedicated to that buffer.  If, however, that
window is the only window on its frame, @code{bury-buffer} displays
another buffer in it and iconifies the frame.

@defun window-dedicated-p &optional window
This function returns non-@code{nil} if @var{window} is dedicated to its
buffer and @code{nil} otherwise.  More precisely, the return value is
the value assigned by the last call of @code{set-window-dedicated-p} for
@var{window}, or @code{nil} if that function was never called with
@var{window} as its argument.  The default for @var{window} is the
selected window.
@end defun

@defun set-window-dedicated-p window flag
This function marks @var{window} as dedicated to its buffer if
@var{flag} is non-@code{nil}, and non-dedicated otherwise.

As a special case, if @var{flag} is @code{t}, @var{window} becomes
@dfn{strongly} dedicated to its buffer.  @code{set-window-buffer}
signals an error when the window it acts upon is strongly dedicated to
its buffer and does not already display the buffer it is asked to
display.  Other functions do not treat @code{t} differently from any
non-@code{nil} value.
@end defun


@node Quitting Windows
@section Quitting Windows

When you want to get rid of a window used for displaying a buffer, you
can call @code{delete-window} or @code{delete-windows-on}
(@pxref{Deleting Windows}) to remove that window from its frame.  If the
buffer is shown on a separate frame, you might want to call
@code{delete-frame} (@pxref{Deleting Frames}) instead.  If, on the other
hand, a window has been reused for displaying the buffer, you might
prefer showing the buffer previously shown in that window, by calling the
function @code{switch-to-prev-buffer} (@pxref{Window History}).
Finally, you might want to either bury (@pxref{The Buffer List}) or kill
(@pxref{Killing Buffers}) the window's buffer.

   The following command uses information on how the window for
displaying the buffer was obtained in the first place, thus attempting
to automate the above decisions for you.

@deffn Command quit-window &optional kill window
This command quits @var{window} and buries its buffer.  The argument
@var{window} must be a live window and defaults to the selected one.
With prefix argument @var{kill} non-@code{nil}, it kills the buffer
instead of burying it.  It calls the function @code{quit-restore-window}
described next to deal with the window and its buffer.
@end deffn

@defun quit-restore-window &optional window bury-or-kill
This function tries to restore the state of @var{window} that existed
before its buffer was displayed in it.  The optional argument
@var{window} must be a live window and defaults to the selected one.

If @var{window} was created specially for displaying its buffer, this
function deletes @var{window} provided its frame contains at least one
other live window.  If @var{window} is the only window on its frame and
there are other frames on the frame's terminal, the value of the
optional argument @var{bury-or-kill} determines how to proceed with the
window.  If @var{bury-or-kill} equals @code{kill}, the frame is deleted
unconditionally.  Otherwise, the fate of the frame is determined by
calling @code{frame-auto-hide-function} (see below) with that frame as
sole argument.

Otherwise, this function tries to redisplay the buffer previously shown
in @var{window}.  It also tries to restore the window start
(@pxref{Window Start and End}) and point (@pxref{Window Point})
positions of the previously shown buffer.  If, in addition,
@var{window}'s buffer was temporarily resized, this function will also
try to restore the original height of @var{window}.

The cases described so far require that the buffer shown in @var{window}
is still the buffer displayed by the last buffer display function for
this window.  If another buffer has been shown in the meantime, or the
buffer previously shown no longer exists, this function calls
@code{switch-to-prev-buffer} (@pxref{Window History}) to show some other
buffer instead.

The optional argument @var{bury-or-kill} specifes how to deal with
@var{window}'s buffer.  The following values are handled:

@table @code
@item nil
This means to not deal with the buffer in any particular way.  As a
consequence, if @var{window} is not deleted, invoking
@code{switch-to-prev-buffer} will usually show the buffer again.

@item append
This means that if @var{window} is not deleted, its buffer is moved to
the end of @var{window}'s list of previous buffers, so it's less likely
that a future invocation of @code{switch-to-prev-buffer} will switch to
it.  Also, it moves the buffer to the end of the frame's buffer list.

@item bury
This means that if @var{window} is not deleted, its buffer is removed
from @var{window}'s list of previous buffers.  Also, it moves the buffer
to the end of the frame's buffer list.  This value provides the most
reliable remedy to not have @code{switch-to-prev-buffer} switch to this
buffer again without killing the buffer.

@item kill
This means to kill @var{window}'s buffer.
@end table

@code{quit-restore-window} bases its decisions on information stored in
@var{window}'s @code{quit-restore} window parameter (@pxref{Window
Parameters}), and resets that parameter to @code{nil} after it's done.
@end defun

The following option specifies how to deal with a frame containing just
one window that should be either quit, or whose buffer should be buried.

@defopt frame-auto-hide-function
The function specified by this option is called to automatically hide
frames.  This function is called with one argument---a frame.

The function specified here is called by @code{bury-buffer} (@pxref{The
Buffer List}) when the selected window is dedicated and shows the buffer
to bury.  It is also called by @code{quit-restore-window} (see above)
when the frame of the window to quit has been specially created for
displaying that window's buffer and the buffer is not killed.

The default is to call @code{iconify-frame} (@pxref{Visibility of
Frames}).  Alternatively, you may specify either @code{delete-frame}
(@pxref{Deleting Frames}) to remove the frame from its display,
@code{ignore} to leave the frame unchanged, or any other function that
can take a frame as its sole argument.

Note that the function specified by this option is called only if the
specified frame contains just one live window and there is at least one
other frame on the same terminal.
@end defopt


@node Window Point
@section Windows and Point
@cindex window position
@cindex window point
@cindex position in window
@cindex point in window

  Each window has its own value of point (@pxref{Point}), independent of
the value of point in other windows displaying the same buffer.  This
makes it useful to have multiple windows showing one buffer.

@itemize @bullet
@item
The window point is established when a window is first created; it is
initialized from the buffer's point, or from the window point of another
window opened on the buffer if such a window exists.

@item
Selecting a window sets the value of point in its buffer from the
window's value of point.  Conversely, deselecting a window sets the
window's value of point from that of the buffer.  Thus, when you switch
between windows that display a given buffer, the point value for the
selected window is in effect in the buffer, while the point values for
the other windows are stored in those windows.

@item
As long as the selected window displays the current buffer, the window's
point and the buffer's point always move together; they remain equal.
@end itemize

@cindex cursor
   As far as the user is concerned, point is where the cursor is, and
when the user switches to another buffer, the cursor jumps to the
position of point in that buffer.

@defun window-point &optional window
This function returns the current position of point in @var{window}.
For a nonselected window, this is the value point would have (in that
window's buffer) if that window were selected.  The default for
@var{window} is the selected window.

When @var{window} is the selected window, the value returned is the
value of point in that window's buffer.  Strictly speaking, it would be
more correct to return the ``top-level'' value of point, outside of any
@code{save-excursion} forms.  But that value is hard to find.
@end defun

@defun set-window-point window position
This function positions point in @var{window} at position
@var{position} in @var{window}'s buffer.  It returns @var{position}.

If @var{window} is selected, this simply does @code{goto-char} in
@var{window}'s buffer.
@end defun

@defvar window-point-insertion-type
This variable specifies the marker insertion type (@pxref{Marker
Insertion Types}) of @code{window-point}.  The default is @code{nil},
so @code{window-point} will stay behind text inserted there.
@end defvar

@node Window Start and End
@section The Window Start and End Positions
@cindex window start position

  Each window maintains a marker used to keep track of a buffer position
that specifies where in the buffer display should start.  This position
is called the @dfn{display-start} position of the window (or just the
@dfn{start}).  The character after this position is the one that appears
at the upper left corner of the window.  It is usually, but not
inevitably, at the beginning of a text line.

  After switching windows or buffers, and in some other cases, if the
window start is in the middle of a line, Emacs adjusts the window
start to the start of a line.  This prevents certain operations from
leaving the window start at a meaningless point within a line.  This
feature may interfere with testing some Lisp code by executing it
using the commands of Lisp mode, because they trigger this
readjustment.  To test such code, put it into a command and bind the
command to a key.

@defun window-start &optional window
@cindex window top line
This function returns the display-start position of window
@var{window}.  If @var{window} is @code{nil}, the selected window is
used.

When you create a window, or display a different buffer in it, the
display-start position is set to a display-start position recently used
for the same buffer, or to @code{point-min} if the buffer doesn't have
any.

Redisplay updates the window-start position (if you have not specified
it explicitly since the previous redisplay)---to make sure point appears
on the screen.  Nothing except redisplay automatically changes the
window-start position; if you move point, do not expect the window-start
position to change in response until after the next redisplay.
@end defun

@cindex window end position
@defun window-end &optional window update
This function returns the position where display of its buffer ends in
@var{window}.  The default for @var{window} is the selected window.

Simply changing the buffer text or moving point does not update the
value that @code{window-end} returns.  The value is updated only when
Emacs redisplays and redisplay completes without being preempted.

If the last redisplay of @var{window} was preempted, and did not finish,
Emacs does not know the position of the end of display in that window.
In that case, this function returns @code{nil}.

If @var{update} is non-@code{nil}, @code{window-end} always returns an
up-to-date value for where display ends, based on the current
@code{window-start} value.  If a previously saved value of that position
is still valid, @code{window-end} returns that value; otherwise it
computes the correct value by scanning the buffer text.

Even if @var{update} is non-@code{nil}, @code{window-end} does not
attempt to scroll the display if point has moved off the screen, the
way real redisplay would do.  It does not alter the
@code{window-start} value.  In effect, it reports where the displayed
text will end if scrolling is not required.
@end defun

@defun set-window-start window position &optional noforce
This function sets the display-start position of @var{window} to
@var{position} in @var{window}'s buffer.  It returns @var{position}.

The display routines insist that the position of point be visible when a
buffer is displayed.  Normally, they change the display-start position
(that is, scroll the window) whenever necessary to make point visible.
However, if you specify the start position with this function using
@code{nil} for @var{noforce}, it means you want display to start at
@var{position} even if that would put the location of point off the
screen.  If this does place point off screen, the display routines move
point to the left margin on the middle line in the window.

For example, if point @w{is 1} and you set the start of the window
@w{to 37}, the start of the next line, point will be ``above'' the top
of the window.  The display routines will automatically move point if
it is still 1 when redisplay occurs.  Here is an example:

@example
@group
;; @r{Here is what @samp{foo} looks like before executing}
;;   @r{the @code{set-window-start} expression.}
@end group

@group
---------- Buffer: foo ----------
@point{}This is the contents of buffer foo.
2
3
4
5
6
---------- Buffer: foo ----------
@end group

@group
(set-window-start
 (selected-window)
 (save-excursion
   (goto-char 1)
   (forward-line 1)
   (point)))
@result{} 37
@end group

@group
;; @r{Here is what @samp{foo} looks like after executing}
;;   @r{the @code{set-window-start} expression.}
---------- Buffer: foo ----------
2
3
@point{}4
5
6
---------- Buffer: foo ----------
@end group
@end example

If @var{noforce} is non-@code{nil}, and @var{position} would place point
off screen at the next redisplay, then redisplay computes a new window-start
position that works well with point, and thus @var{position} is not used.
@end defun

@defun pos-visible-in-window-p &optional position window partially
This function returns non-@code{nil} if @var{position} is within the
range of text currently visible on the screen in @var{window}.  It
returns @code{nil} if @var{position} is scrolled vertically out of view.
Locations that are partially obscured are not considered visible unless
@var{partially} is non-@code{nil}.  The argument @var{position} defaults
to the current position of point in @var{window}; @var{window}, to the
selected window.  If @var{position} is @code{t}, that means to check the
last visible position in @var{window}.

This function considers only vertical scrolling.  If @var{position} is
out of view only because @var{window} has been scrolled horizontally,
@code{pos-visible-in-window-p} returns non-@code{nil} anyway.
@xref{Horizontal Scrolling}.

If @var{position} is visible, @code{pos-visible-in-window-p} returns
@code{t} if @var{partially} is @code{nil}; if @var{partially} is
non-@code{nil}, and the character following @var{position} is fully
visible, it returns a list of the form @code{(@var{x} @var{y})}, where
@var{x} and @var{y} are the pixel coordinates relative to the top left
corner of the window; otherwise it returns an extended list of the form
@code{(@var{x} @var{y} @var{rtop} @var{rbot} @var{rowh} @var{vpos})},
where @var{rtop} and @var{rbot} specify the number of off-window pixels
at the top and bottom of the row at @var{position}, @var{rowh} specifies
the visible height of that row, and @var{vpos} specifies the vertical
position (zero-based row number) of that row.

Here is an example:

@example
@group
;; @r{If point is off the screen now, recenter it now.}
(or (pos-visible-in-window-p
     (point) (selected-window))
    (recenter 0))
@end group
@end example
@end defun

@defun window-line-height &optional line window
This function returns the height of text line @var{line} in
@var{window}.  If @var{line} is one of @code{header-line} or
@code{mode-line}, @code{window-line-height} returns information about
the corresponding line of the window.  Otherwise, @var{line} is a text
line number starting from 0.  A negative number counts from the end of
the window.  The default for @var{line} is the current line in
@var{window}; the default for @var{window} is the selected window.

If the display is not up to date, @code{window-line-height} returns
@code{nil}.  In that case, @code{pos-visible-in-window-p} may be used
to obtain related information.

If there is no line corresponding to the specified @var{line},
@code{window-line-height} returns @code{nil}.  Otherwise, it returns
a list @code{(@var{height} @var{vpos} @var{ypos} @var{offbot})},
where @var{height} is the height in pixels of the visible part of the
line, @var{vpos} and @var{ypos} are the vertical position in lines and
pixels of the line relative to the top of the first text line, and
@var{offbot} is the number of off-window pixels at the bottom of the
text line.  If there are off-window pixels at the top of the (first)
text line, @var{ypos} is negative.
@end defun

@node Textual Scrolling
@section Textual Scrolling
@cindex textual scrolling
@cindex scrolling textually

  @dfn{Textual scrolling} means moving the text up or down through a
window.  It works by changing the window's display-start location.  It
may also change the value of @code{window-point} to keep point on the
screen (@pxref{Window Point}).

  The basic textual scrolling functions are @code{scroll-up} (which
scrolls forward) and @code{scroll-down} (which scrolls backward).  In
these function names, ``up'' and ``down'' refer to the direction of
motion of the buffer text relative to the window.  Imagine that the
text is written on a long roll of paper and that the scrolling
commands move the paper up and down.  Thus, if you are looking at the
middle of a buffer and repeatedly call @code{scroll-down}, you will
eventually see the beginning of the buffer.

  Unfortunately, this sometimes causes confusion, because some people
tend to think in terms of the opposite convention: they
imagine the window moving over text that remains in place, so that
``down'' commands take you to the end of the buffer.  This convention
is consistent with fact that such a command is bound to a key named
@key{PageDown} on modern keyboards.
@ignore
We have not switched to this convention as that is likely to break
existing Emacs Lisp code.
@end ignore

  Textual scrolling functions (aside from @code{scroll-other-window})
have unpredictable results if the current buffer is not the one
displayed in the selected window.  @xref{Current Buffer}.

  If the window contains a row taller than the height of the window
(for example in the presence of a large image), the scroll functions
will adjust the window's vertical scroll position to scroll the
partially visible row.  Lisp callers can disable this feature by
binding the variable @code{auto-window-vscroll} to @code{nil}
(@pxref{Vertical Scrolling}).

@deffn Command scroll-up &optional count
This function scrolls forward by @var{count} lines in the selected
window.

If @var{count} is negative, it scrolls backward instead.  If
@var{count} is @code{nil} (or omitted), the distance scrolled is
@code{next-screen-context-lines} lines less than the height of the
window's text area.

If the selected window cannot be scrolled any further, this function
signals an error.  Otherwise, it returns @code{nil}.
@end deffn

@deffn Command scroll-down &optional count
This function scrolls backward by @var{count} lines in the selected
window.

If @var{count} is negative, it scrolls forward instead.  In other
respects, it behaves the same way as @code{scroll-up} does.
@end deffn

@deffn Command scroll-up-command &optional count
This behaves like @code{scroll-up}, except that if the selected window
cannot be scrolled any further and the value of the variable
@code{scroll-error-top-bottom} is @code{t}, it tries to move to the
end of the buffer instead.  If point is already there, it signals an
error.
@end deffn

@deffn Command scroll-down-command &optional count
This behaves like @code{scroll-down}, except that if the selected
window cannot be scrolled any further and the value of the variable
@code{scroll-error-top-bottom} is @code{t}, it tries to move to the
beginning of the buffer instead.  If point is already there, it
signals an error.
@end deffn

@deffn Command scroll-other-window &optional count
This function scrolls the text in another window upward @var{count}
lines.  Negative values of @var{count}, or @code{nil}, are handled
as in @code{scroll-up}.

You can specify which buffer to scroll by setting the variable
@code{other-window-scroll-buffer} to a buffer.  If that buffer isn't
already displayed, @code{scroll-other-window} displays it in some
window.

When the selected window is the minibuffer, the next window is normally
the leftmost one immediately above it.  You can specify a different
window to scroll, when the minibuffer is selected, by setting the variable
@code{minibuffer-scroll-window}.  This variable has no effect when any
other window is selected.  When it is non-@code{nil} and the
minibuffer is selected, it takes precedence over
@code{other-window-scroll-buffer}.  @xref{Definition of
minibuffer-scroll-window}.

When the minibuffer is active, it is the next window if the selected
window is the one at the bottom right corner.  In this case,
@code{scroll-other-window} attempts to scroll the minibuffer.  If the
minibuffer contains just one line, it has nowhere to scroll to, so the
line reappears after the echo area momentarily displays the message
@samp{End of buffer}.
@end deffn

@defvar other-window-scroll-buffer
If this variable is non-@code{nil}, it tells @code{scroll-other-window}
which buffer's window to scroll.
@end defvar

@defopt scroll-margin
This option specifies the size of the scroll margin---a minimum number
of lines between point and the top or bottom of a window.  Whenever
point gets within this many lines of the top or bottom of the window,
redisplay scrolls the text automatically (if possible) to move point
out of the margin, closer to the center of the window.
@end defopt

@defopt scroll-conservatively
This variable controls how scrolling is done automatically when point
moves off the screen (or into the scroll margin).  If the value is a
positive integer @var{n}, then redisplay scrolls the text up to
@var{n} lines in either direction, if that will bring point back into
proper view.  This behavior is called @dfn{conservative scrolling}.
Otherwise, scrolling happens in the usual way, under the control of
other variables such as @code{scroll-up-aggressively} and
@code{scroll-down-aggressively}.

The default value is zero, which means that conservative scrolling
never happens.
@end defopt

@defopt scroll-down-aggressively
The value of this variable should be either @code{nil} or a fraction
@var{f} between 0 and 1.  If it is a fraction, that specifies where on
the screen to put point when scrolling down.  More precisely, when a
window scrolls down 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.

A value of @code{nil} is equivalent to .5, since its effect is to center
point.  This variable automatically becomes buffer-local when set in any
fashion.
@end defopt

@defopt scroll-up-aggressively
Likewise, for scrolling up.  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 scrolls more aggressively.
@end defopt

@defopt scroll-step
This variable is an older variant of @code{scroll-conservatively}.
The difference is that if its value is @var{n}, that permits scrolling
only by precisely @var{n} lines, not a smaller number.  This feature
does not work with @code{scroll-margin}.  The default value is zero.
@end defopt

@cindex @code{scroll-command} property
@defopt scroll-preserve-screen-position
If this option is @code{t}, whenever a scrolling command moves point
off-window, Emacs tries to adjust point to keep the cursor at its old
vertical position in the window, rather than the window edge.

If the value is non-@code{nil} and not @code{t}, Emacs adjusts point
to keep the cursor at the same vertical position, even if the
scrolling command didn't move point off-window.

This option affects all scroll commands that have a non-@code{nil}
@code{scroll-command} symbol property.
@end defopt

@defopt next-screen-context-lines
The value of this variable is the number of lines of continuity to
retain when scrolling by full screens.  For example, @code{scroll-up}
with an argument of @code{nil} scrolls so that this many lines at the
bottom of the window appear instead at the top.  The default value is
@code{2}.
@end defopt

@defopt scroll-error-top-bottom
If this option is @code{nil} (the default), @code{scroll-up-command}
and @code{scroll-down-command} simply signal an error when no more
scrolling is possible.

If the value is @code{t}, these commands instead move point to the
beginning or end of the buffer (depending on scrolling direction);
only if point is already on that position do they signal an error.
@end defopt

@deffn Command recenter &optional count
@cindex centering point
This function scrolls the text in the selected window so that point is
displayed at a specified vertical position within the window.  It does
not ``move point'' with respect to the text.

If @var{count} is a non-negative number, that puts the line containing
point @var{count} lines down from the top of the window.  If
@var{count} is a negative number, then it counts upward from the
bottom of the window, so that @minus{}1 stands for the last usable
line in the window.

If @var{count} is @code{nil} (or a non-@code{nil} list),
@code{recenter} puts the line containing point in the middle of the
window.  If @var{count} is @code{nil}, this function may redraw the
frame, according to the value of @code{recenter-redisplay}.

When @code{recenter} is called interactively, @var{count} is the raw
prefix argument.  Thus, typing @kbd{C-u} as the prefix sets the
@var{count} to a non-@code{nil} list, while typing @kbd{C-u 4} sets
@var{count} to 4, which positions the current line four lines from the
top.

With an argument of zero, @code{recenter} positions the current line at
the top of the window.  The command @code{recenter-top-bottom} offers
a more convenient way to achieve this.
@end deffn

@defopt recenter-redisplay
If this variable is non-@code{nil}, calling @code{recenter} with a
@code{nil} argument redraws the frame.  The default value is
@code{tty}, which means only redraw the frame if it is a tty frame.
@end defopt

@deffn Command recenter-top-bottom &optional count
This command, which is the default binding for @kbd{C-l}, acts like
@code{recenter}, except if called with no argument.  In that case,
successive calls place point according to the cycling order defined
by the variable @code{recenter-positions}.
@end deffn

@defopt recenter-positions
This variable controls how @code{recenter-top-bottom} behaves when
called with no argument.  The default value is @code{(middle top
bottom)}, which means that successive calls of
@code{recenter-top-bottom} with no argument cycle between placing
point at the middle, top, and bottom of the window.
@end defopt


@node Vertical Scrolling
@section Vertical Fractional Scrolling
@cindex vertical fractional scrolling
@cindex vertical scroll position

   @dfn{Vertical fractional scrolling} means shifting text in a window
up or down by a specified multiple or fraction of a line.  Each window
has a @dfn{vertical scroll position}, which is a number, never less than
zero.  It specifies how far to raise the contents of the window.
Raising the window contents generally makes all or part of some lines
disappear off the top, and all or part of some other lines appear at the
bottom.  The usual value is zero.

   The vertical scroll position is measured in units of the normal line
height, which is the height of the default font.  Thus, if the value is
.5, that means the window contents are scrolled up half the normal line
height.  If it is 3.3, that means the window contents are scrolled up
somewhat over three times the normal line height.

   What fraction of a line the vertical scrolling covers, or how many
lines, depends on what the lines contain.  A value of .5 could scroll a
line whose height is very short off the screen, while a value of 3.3
could scroll just part of the way through a tall line or an image.

@defun window-vscroll &optional window pixels-p
This function returns the current vertical scroll position of
@var{window}.  The default for @var{window} is the selected window.
If @var{pixels-p} is non-@code{nil}, the return value is measured in
pixels, rather than in units of the normal line height.

@example
@group
(window-vscroll)
     @result{} 0
@end group
@end example
@end defun

@defun set-window-vscroll window lines &optional pixels-p
This function sets @var{window}'s vertical scroll position to
@var{lines}.  If @var{window} is @code{nil}, the selected window is
used.  The argument @var{lines} should be zero or positive; if not, it
is taken as zero.


The actual vertical scroll position must always correspond
to an integral number of pixels, so the value you specify
is rounded accordingly.

The return value is the result of this rounding.

@example
@group
(set-window-vscroll (selected-window) 1.2)
     @result{} 1.13
@end group
@end example

If @var{pixels-p} is non-@code{nil}, @var{lines} specifies a number of
pixels.  In this case, the return value is @var{lines}.
@end defun

@defvar auto-window-vscroll
If this variable is non-@code{nil}, the line-move, scroll-up, and
scroll-down functions will automatically modify the vertical scroll
position to scroll through display rows that are taller than the height
of the window, for example in the presence of large images.
@end defvar

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

  @dfn{Horizontal scrolling} means shifting the image in the window left
or right by a specified multiple of the normal character width.  Each
window has a @dfn{horizontal scroll position}, which is a number, never
less than zero.  It specifies how far to shift the contents left.
Shifting the window contents left generally makes all or part of some
characters disappear off the left, and all or part of some other
characters appear at the right.  The usual value is zero.

  The horizontal scroll position is measured in units of the normal
character width, which is the width of space in the default font.  Thus,
if the value is 5, that means the window contents are scrolled left by 5
times the normal character width.  How many characters actually
disappear off to the left depends on their width, and could vary from
line to line.

  Because we read from side to side in the ``inner loop'', and from top
to bottom in the ``outer loop'', the effect of horizontal scrolling is
not like that of textual or vertical scrolling.  Textual scrolling
involves selection of a portion of text to display, and vertical
scrolling moves the window contents contiguously; but horizontal
scrolling causes part of @emph{each line} to go off screen.

  Usually, no horizontal scrolling is in effect; then the leftmost
column is at the left edge of the window.  In this state, scrolling to
the right is meaningless, since there is no data to the left of the edge
to be revealed by it; so this is not allowed.  Scrolling to the left is
allowed; it scrolls the first columns of text off the edge of the window
and can reveal additional columns on the right that were truncated
before.  Once a window has a nonzero amount of leftward horizontal
scrolling, you can scroll it back to the right, but only so far as to
reduce the net horizontal scroll to zero.  There is no limit to how far
left you can scroll, but eventually all the text will disappear off the
left edge.

@vindex auto-hscroll-mode
  If @code{auto-hscroll-mode} is set, redisplay automatically alters
the horizontal scrolling of a window as necessary to ensure that point
is always visible.  However, you can still set the horizontal
scrolling value explicitly.  The value you specify serves as a lower
bound for automatic scrolling, i.e. automatic scrolling will not
scroll a window to a column less than the specified one.

@deffn Command scroll-left &optional count set-minimum
This function scrolls the selected window @var{count} columns to the
left (or to the right if @var{count} is negative).  The default
for @var{count} is the window width, minus 2.

The return value is the total amount of leftward horizontal scrolling in
effect after the change---just like the value returned by
@code{window-hscroll} (below).

Once you scroll a window as far right as it can go, back to its normal
position where the total leftward scrolling is zero, attempts to scroll
any farther right have no effect.

If @var{set-minimum} is non-@code{nil}, the new scroll amount becomes
the lower bound for automatic scrolling; that is, automatic scrolling
will not scroll a window to a column less than the value returned by
this function.  Interactive calls pass non-@code{nil} for
@var{set-minimum}.
@end deffn

@deffn Command scroll-right &optional count set-minimum
This function scrolls the selected window @var{count} columns to the
right (or to the left if @var{count} is negative).  The default
for @var{count} is the window width, minus 2.  Aside from the direction
of scrolling, this works just like @code{scroll-left}.
@end deffn

@defun window-hscroll &optional window
This function returns the total leftward horizontal scrolling of
@var{window}---the number of columns by which the text in @var{window}
is scrolled left past the left margin.  The default for
@var{window} is the selected window.

The return value is never negative.  It is zero when no horizontal
scrolling has been done in @var{window} (which is usually the case).


@example
@group
(window-hscroll)
     @result{} 0
@end group
@group
(scroll-left 5)
     @result{} 5
@end group
@group
(window-hscroll)
     @result{} 5
@end group
@end example
@end defun

@defun set-window-hscroll window columns
This function sets horizontal scrolling of @var{window}.  The value of
@var{columns} specifies the amount of scrolling, in terms of columns
from the left margin.  The argument @var{columns} should be zero or
positive; if not, it is taken as zero.  Fractional values of
@var{columns} are not supported at present.

Note that @code{set-window-hscroll} may appear not to work if you test
it by evaluating a call with @kbd{M-:} in a simple way.  What happens
is that the function sets the horizontal scroll value and returns, but
then redisplay adjusts the horizontal scrolling to make point visible,
and this overrides what the function did.  You can observe the
function's effect if you call it while point is sufficiently far from
the left margin that it will remain visible.

The value returned is @var{columns}.

@example
@group
(set-window-hscroll (selected-window) 10)
     @result{} 10
@end group
@end example
@end defun

   Here is how you can determine whether a given position @var{position}
is off the screen due to horizontal scrolling:

@example
@group
(defun hscroll-on-screen (window position)
  (save-excursion
    (goto-char position)
    (and
     (>= (- (current-column) (window-hscroll window)) 0)
     (< (- (current-column) (window-hscroll window))
        (window-width window)))))
@end group
@end example

@node Coordinates and Windows
@section Coordinates and Windows
@cindex frame-relative coordinate
@cindex coordinate, relative to frame
@cindex window position

  This section describes functions that report the position of a
window.  Most of these functions report positions relative to the
window's frame.  In this case, the coordinate origin @samp{(0,0)} lies
near the upper left corner of the frame.  For technical reasons, on
graphical displays the origin is not located at the exact corner of
the graphical window as it appears on the screen.  If Emacs is built
with the GTK+ toolkit, the origin is at the upper left corner of the
frame area used for displaying Emacs windows, below the title-bar,
GTK+ menu bar, and tool bar (since these are drawn by the window
manager and/or GTK+, not by Emacs).  But if Emacs is not built with
GTK+, the origin is at the upper left corner of the tool bar (since in
this case Emacs itself draws the tool bar).  In both cases, the X and
Y coordinates increase rightward and downward respectively.

  Except where noted, X and Y coordinates are reported in integer
character units, i.e. numbers of lines and columns respectively.  On a
graphical display, each ``line'' and ``column'' corresponds to the
height and width of a default character specified by the frame's
default font.

@defun window-edges &optional window
This function returns a list of the edge coordinates of @var{window}.
If @var{window} is omitted or @code{nil}, it defaults to the selected
window.

The return value has the form @code{(@var{left} @var{top} @var{right}
@var{bottom})}.  These list elements are, respectively, the X
coordinate of the leftmost column occupied by the window, the Y
coordinate of the topmost row, the X coordinate one column to the
right of the rightmost column, and the Y coordinate one row down from
the bottommost row.

Note that these are the actual outer edges of the window, including
any header line, mode line, scroll bar, fringes, and display margins.
On a text terminal, if the window has a neighbor on its right, its
right edge includes the separator line between the window and its
neighbor.
@end defun

@defun window-inside-edges &optional window
This function is similar to @code{window-edges}, but the returned edge
values are for the text area of the window.  They exclude any header
line, mode line, scroll bar, fringes, display margins, and vertical
separator.
@end defun

@defun window-top-line &optional window
This function returns the Y coordinate of the topmost row of
@var{window}, equivalent to the @var{top} entry in the list returned
by @code{window-edges}.
@end defun

@defun window-left-column &optional window
This function returns the X coordinate of the leftmost column of
@var{window}, equivalent to the @var{left} entry in the list returned
by @code{window-edges}.
@end defun

  The following functions can be used to relate a set of
frame-relative coordinates to a window:

@defun window-at x y &optional frame
This function returns the live window at the frame-relative
coordinates @var{x} and @var{y}, on frame @var{frame}.  If there is no
window at that position, the return value is @code{nil}.  If
@var{frame} is omitted or @code{nil}, it defaults to the selected
frame.
@end defun

@defun coordinates-in-window-p coordinates window
This function checks whether a window @var{window} occupies the
frame-relative coordinates @var{coordinates}, and if so, which part of
the window that is.  @var{window} should be a live window.
@var{coordinates} should be a cons cell of the form @code{(@var{x}
. @var{y})}, where @var{x} and @var{y} are frame-relative coordinates.

If there is no window at the specified position, the return value is
@code{nil} .  Otherwise, the return value is one of the following:

@table @code
@item (@var{relx} . @var{rely})
The coordinates are inside @var{window}.  The numbers @var{relx} and
@var{rely} are the equivalent window-relative coordinates for the
specified position, counting from 0 at the top left corner of the
window.

@item mode-line
The coordinates are in the mode line of @var{window}.

@item header-line
The coordinates are in the header line of @var{window}.

@item vertical-line
The coordinates are in the vertical line between @var{window} and its
neighbor to the right.  This value occurs only if the window doesn't
have a scroll bar; positions in a scroll bar are considered outside the
window for these purposes.

@item left-fringe
@itemx right-fringe
The coordinates are in the left or right fringe of the window.

@item left-margin
@itemx right-margin
The coordinates are in the left or right margin of the window.

@item nil
The coordinates are not in any part of @var{window}.
@end table

The function @code{coordinates-in-window-p} does not require a frame as
argument because it always uses the frame that @var{window} is on.
@end defun

  The following functions return window positions in pixels, rather
than character units.  Though mostly useful on graphical displays,
they can also be called on text terminals, where the screen area of
each text character is taken to be ``one pixel''.

@defun window-pixel-edges &optional window
This function returns a list of pixel coordinates for the edges of
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window.

The return value has the form @code{(@var{left} @var{top} @var{right}
@var{bottom})}.  The list elements are, respectively, the X pixel
coordinate of the left window edge, the Y pixel coordinate of the top
edge, one more than the X pixel coordinate of the right edge, and one
more than the Y pixel coordinate of the bottom edge.
@end defun

@defun window-inside-pixel-edges &optional window
This function is like @code{window-pixel-edges}, except that it
returns the pixel coordinates for the edges of the window's text area,
rather than the pixel coordinates for the edges of the window itself.
@var{window} must specify a live window.
@end defun

  The following functions return window positions in pixels, relative
to the display screen rather than the frame:

@defun window-absolute-pixel-edges &optional window
This function is like @code{window-pixel-edges}, except that it
returns the edge pixel coordinates relative to the top left corner of
the display screen.
@end defun

@defun window-inside-absolute-pixel-edges &optional window
This function is like @code{window-inside-pixel-edges}, except that it
returns the edge pixel coordinates relative to the top left corner of
the display screen.  @var{window} must specify a live window.
@end defun

@node Window Configurations
@section Window Configurations
@cindex window configurations
@cindex saving window information

A @dfn{window configuration} records the entire layout of one
frame---all windows, their sizes, which buffers they contain, how those
buffers are scrolled, and their values of point and the mark; also their
fringes, margins, and scroll bar settings.  It also includes the value
of @code{minibuffer-scroll-window}.  As a special exception, the window
configuration does not record the value of point in the selected window
for the current buffer.

  You can bring back an entire frame layout by restoring a previously
saved window configuration.  If you want to record the layout of all
frames instead of just one, use a frame configuration instead of a
window configuration.  @xref{Frame Configurations}.

@defun current-window-configuration &optional frame
This function returns a new object representing @var{frame}'s current
window configuration.  The default for @var{frame} is the selected
frame.  The variable @code{window-persistent-parameters} specifies
which window parameters (if any) are saved by this function.
@xref{Window Parameters}.
@end defun

@defun set-window-configuration configuration
This function restores the configuration of windows and buffers as
specified by @var{configuration}, for the frame that @var{configuration}
was created for.

The argument @var{configuration} must be a value that was previously
returned by @code{current-window-configuration}.  The configuration is
restored in the frame from which @var{configuration} was made, whether
that frame is selected or not.  This always counts as a window size
change and triggers execution of the @code{window-size-change-functions}
(@pxref{Window Hooks}), because @code{set-window-configuration} doesn't
know how to tell whether the new configuration actually differs from the
old one.

If the frame from which @var{configuration} was saved is dead, all this
function does is restore the three variables @code{window-min-height},
@code{window-min-width} and @code{minibuffer-scroll-window}.  In this
case, the function returns @code{nil}.  Otherwise, it returns @code{t}.

Here is a way of using this function to get the same effect
as @code{save-window-excursion}:

@example
@group
(let ((config (current-window-configuration)))
  (unwind-protect
      (progn (split-window-below nil)
             @dots{})
    (set-window-configuration config)))
@end group
@end example
@end defun

@defmac save-window-excursion forms@dots{}
This macro records the window configuration of the selected frame,
executes @var{forms} in sequence, then restores the earlier window
configuration.  The return value is the value of the final form in
@var{forms}.

Most Lisp code should not use this macro; @code{save-selected-window}
is typically sufficient.  In particular, this macro cannot reliably
prevent the code in @var{forms} from opening new windows, because new
windows might be opened in other frames (@pxref{Choosing Window}), and
@code{save-window-excursion} only saves and restores the window
configuration on the current frame.

Do not use this macro in @code{window-size-change-functions}; exiting
the macro triggers execution of @code{window-size-change-functions},
leading to an endless loop.
@end defmac

@defun window-configuration-p object
This function returns @code{t} if @var{object} is a window configuration.
@end defun

@defun compare-window-configurations config1 config2
This function compares two window configurations as regards the
structure of windows, but ignores the values of point and mark and the
saved scrolling positions---it can return @code{t} even if those
aspects differ.

The function @code{equal} can also compare two window configurations; it
regards configurations as unequal if they differ in any respect, even a
saved point or mark.
@end defun

@defun window-configuration-frame config
This function returns the frame for which the window configuration
@var{config} was made.
@end defun

  Other primitives to look inside of window configurations would make
sense, but are not implemented because we did not need them.  See the
file @file{winner.el} for some more operations on windows
configurations.

  The objects returned by @code{current-window-configuration} die
together with the Emacs process.  In order to store a window
configuration on disk and read it back in another Emacs session, you
can use the functions described next.  These functions are also useful
to clone the state of a frame into an arbitrary live window
(@code{set-window-configuration} effectively clones the windows of a
frame into the root window of that very frame only).

@defun window-state-get &optional window writable
This function returns the state of @var{window} as a Lisp object.  The
argument @var{window} must be a valid window and defaults to the root
window of the selected frame.

If the optional argument @var{writable} is non-@code{nil}, this means to
not use markers for sampling positions like @code{window-point} or
@code{window-start}.  This argument should be non-@code{nil} when the
state will be written to disk and read back in another session.

Together, the argument @var{writable} and the variable
@code{window-persistent-parameters} specify which window parameters are
saved by this function.  @xref{Window Parameters}.
@end defun

The value returned by @code{window-state-get} can be used in the same
session to make a clone of a window in another window.  It can be also
written to disk and read back in another session.  In either case, use
the following function to restore the state of the window.

@defun window-state-put state &optional window ignore
This function puts the window state @var{state} into @var{window}.  The
argument @var{state} should be the state of a window returned by an
earlier invocation of @code{window-state-get}, see above.  The optional
argument @var{window} must specify a live window and defaults to the
selected one.

If the optional argument @var{ignore} is non-@code{nil}, it means to ignore
minimum window sizes and fixed-size restrictions.  If @var{ignore}
is @code{safe}, this means windows can get as small as one line
and/or two columns.
@end defun


@node Window Parameters
@section Window Parameters
@cindex window parameters

This section describes how window parameters can be used to associate
additional information with windows.

@defun window-parameter window parameter
This function returns @var{window}'s value for @var{parameter}.  The
default for @var{window} is the selected window.  If @var{window} has no
setting for @var{parameter}, this function returns @code{nil}.
@end defun

@defun window-parameters &optional window
This function returns all parameters of @var{window} and their values.
The default for @var{window} is the selected window.  The return value
is either @code{nil}, or an association list whose elements have the form
@code{(@var{parameter} . @var{value})}.
@end defun

@defun set-window-parameter window parameter value
This function sets @var{window}'s value of @var{parameter} to
@var{value} and returns @var{value}.  The default for @var{window}
is the selected window.
@end defun

By default, the functions that save and restore window configurations or the
states of windows (@pxref{Window Configurations}) do not care about
window parameters.  This means that when you change the value of a
parameter within the body of a @code{save-window-excursion}, the
previous value is not restored when that macro exits.  It also means
that when you restore via @code{window-state-put} a window state saved
earlier by @code{window-state-get}, all cloned windows have their
parameters reset to @code{nil}.  The following variable allows you to
override the standard behavior:

@defvar window-persistent-parameters
This variable is an alist specifying which parameters get saved by
@code{current-window-configuration} and @code{window-state-get}, and
subsequently restored by @code{set-window-configuration} and
@code{window-state-put}.  @xref{Window Configurations}.

The @sc{car} of each entry of this alist is a symbol specifying the
parameter.  The @sc{cdr} should be one of the following:

@table @asis
@item @code{nil}
This value means the parameter is saved neither by
@code{window-state-get} nor by @code{current-window-configuration}.

@item @code{t}
This value specifies that the parameter is saved by
@code{current-window-configuration} and (provided its @var{writable}
argument is @code{nil}) by @code{window-state-get}.

@item @code{writable}
This means that the parameter is saved unconditionally by both
@code{current-window-configuration} and @code{window-state-get}.  This
value should not be used for parameters whose values do not have a read
syntax.  Otherwise, invoking @code{window-state-put} in another session
may fail with an @code{invalid-read-syntax} error.
@end table
@end defvar

Some functions (notably @code{delete-window},
@code{delete-other-windows} and @code{split-window}), may behave specially
when their @var{window} argument has a parameter set.  You can override
such special behavior by binding the following variable to a
non-@code{nil} value:

@defvar ignore-window-parameters
If this variable is non-@code{nil}, some standard functions do not
process window parameters.  The functions currently affected by this are
@code{split-window}, @code{delete-window}, @code{delete-other-windows},
and @code{other-window}.

An application can bind this variable to a non-@code{nil} value around
calls to these functions.  If it does so, the application is fully
responsible for correctly assigning the parameters of all involved
windows when exiting that function.
@end defvar

The following parameters are currently used by the window management
code:

@table @asis
@item @code{delete-window}
This parameter affects the execution of @code{delete-window}
(@pxref{Deleting Windows}).

@item @code{delete-other-windows}
This parameter affects the execution of @code{delete-other-windows}
(@pxref{Deleting Windows}).

@item @code{split-window}
This parameter affects the execution of @code{split-window}
(@pxref{Splitting Windows}).

@item @code{other-window}
This parameter affects the execution of @code{other-window}
(@pxref{Cyclic Window Ordering}).

@item @code{no-other-window}
This parameter marks the window as not selectable by @code{other-window}
(@pxref{Cyclic Window Ordering}).

@item @code{clone-of}
This parameter specifies the window that this one has been cloned
from.  It is installed by @code{window-state-get} (@pxref{Window
Configurations}).

@item @code{quit-restore}
This parameter is installed by the buffer display functions
(@pxref{Choosing Window}) and consulted by @code{quit-restore-window}
(@pxref{Quitting Windows}).  It contains four elements:

The first element is one of the symbols @code{window} - meaning that the
window has been specially created by @code{display-buffer}, @code{frame}
- a separate frame has been created, @code{same} - the window has
displayed the same buffer before, or @code{other} - the window showed
another buffer before.

The second element is either one of the symbols @code{window} or
@code{frame}, or a list whose elements are the buffer shown in the
window before, that buffer's window start and window point positions,
and the window's height at that time.

The third element is the window selected at the time the parameter was
created.  The function @code{quit-restore-window} tries to reselect that
window when it deletes the window passed to it as argument.

The fourth element is the buffer whose display caused the creation of
this parameter.  @code{quit-restore-window} deletes the specified window
only if it still shows that buffer.
@end table

There are additional parameters @code{window-atom} and @code{window-side};
these are reserved and should not be used by applications.


@node Window Hooks
@section Hooks for Window Scrolling and Changes
@cindex hooks for window operations

This section describes how a Lisp program can take action whenever a
window displays a different part of its buffer or a different buffer.
There are three actions that can change this: scrolling the window,
switching buffers in the window, and changing the size of the window.
The first two actions run @code{window-scroll-functions}; the last runs
@code{window-size-change-functions}.

@defvar window-scroll-functions
This variable holds a list of functions that Emacs should call before
redisplaying a window with scrolling.  Displaying a different buffer in
the window also runs these functions.

This variable is not a normal hook, because each function is called with
two arguments: the window, and its new display-start position.

These functions must take care when using @code{window-end}
(@pxref{Window Start and End}); if you need an up-to-date value, you
must use the @var{update} argument to ensure you get it.

@strong{Warning:} don't use this feature to alter the way the window
is scrolled.  It's not designed for that, and such use probably won't
work.
@end defvar

@defvar window-size-change-functions
This variable holds a list of functions to be called if the size of any
window changes for any reason.  The functions are called just once per
redisplay, and just once for each frame on which size changes have
occurred.

Each function receives the frame as its sole argument.  There is no
direct way to find out which windows on that frame have changed size, or
precisely how.  However, if a size-change function records, at each
call, the existing windows and their sizes, it can also compare the
present sizes and the previous sizes.

Creating or deleting windows counts as a size change, and therefore
causes these functions to be called.  Changing the frame size also
counts, because it changes the sizes of the existing windows.

You may use @code{save-selected-window} in these functions
(@pxref{Selecting Windows}).  However, do not use
@code{save-window-excursion} (@pxref{Window Configurations}); exiting
that macro counts as a size change, which would cause these functions
to be called over and over.
@end defvar

@defvar window-configuration-change-hook
A normal hook that is run every time you change the window configuration
of an existing frame.  This includes splitting or deleting windows,
changing the sizes of windows, or displaying a different buffer in a
window.

The buffer-local part of this hook is run once for each window on the
affected frame, with the relevant window selected and its buffer
current.  The global part is run once for the modified frame, with that
frame selected.
@end defvar

  In addition, you can use @code{jit-lock-register} to register a Font
Lock fontification function, which will be called whenever parts of a
buffer are (re)fontified because a window was scrolled or its size
changed.  @xref{Other Font Lock Variables}.