28. Windows

This chapter describes most of the functions and variables related to Emacs windows. See 38. Emacs Display, for information on how text is displayed in windows.

28.1 Basic Concepts of Emacs Windows

A window in Emacs is the physical area of the screen in which a buffer is displayed. The term is also used to refer to a Lisp object that represents that screen area in Emacs Lisp. It should be clear from the context which is meant.

Emacs groups windows into frames. A frame represents an area of screen available for Emacs to use. Each frame always contains at least one window, but you can subdivide it vertically or horizontally into multiple nonoverlapping Emacs windows.

In each frame, at any time, one and only one window is designated as selected within the frame. The frame's cursor appears in that window. At any time, one frame is the selected frame; and the window selected within that frame is the selected window. The selected window's buffer is usually the current buffer (except when set-buffer has been used). See section 27.2 The Current Buffer.

For practical purposes, a window exists only while it is displayed in a frame. Once removed from the frame, the window is effectively deleted and should not be used, even though there may still be references to it from other Lisp objects. Restoring a saved window configuration is the only way for a window no longer on the screen to come back to life. (See section 28.3 Deleting Windows.)

Each window has the following attributes:

• containing frame

• window height

• window width

• window edges with respect to the screen or frame

• the buffer it displays

• position within the buffer at the upper left of the window

• amount of horizontal scrolling, in columns

• point

• the mark

• how recently the window was selected

Users create multiple windows so they can look at several buffers at once. Lisp libraries use multiple windows for a variety of reasons, but most often to display related information. In Rmail, for example, you can move through a summary buffer in one window while the other window shows messages one at a time as they are reached.

The meaning of "window" in Emacs is similar to what it means in the context of general-purpose window systems such as X, but not identical. The X Window System places X windows on the screen; Emacs uses one or more X windows as frames, and subdivides them into Emacs windows. When you use Emacs on a character-only terminal, Emacs treats the whole terminal screen as one frame.

Most window systems support arbitrarily located overlapping windows. In contrast, Emacs windows are tiled; they never overlap, and together they fill the whole screen or frame. Because of the way in which Emacs creates new windows and resizes them, not all conceivable tilings of windows on an Emacs frame are actually possible. See section 28.2 Splitting Windows, and 28.14 The Size of a Window.

See section 38. Emacs Display, for information on how the contents of the window's buffer are displayed in the window.

Function: windowp object

This function returns t if object is a window.

28.2 Splitting Windows

The functions described here are the primitives used to split a window into two windows. Two higher level functions sometimes split a window, but not always: pop-to-buffer and display-buffer (see section 28.7 Displaying Buffers in Windows).

The functions described here do not accept a buffer as an argument. The two "halves" of the split window initially display the same buffer previously visible in the window that was split.

Command: split-window &optional window size horizontal

This function splits window into two windows. The original window window remains the selected window, but occupies only part of its former screen area. The rest is occupied by a newly created window which is returned as the value of this function.

If horizontal is non-nil, then window splits into two side by side windows. The original window window keeps the leftmost size columns, and gives the rest of the columns to the new window. Otherwise, it splits into windows one above the other, and window keeps the upper size lines and gives the rest of the lines to the new window. The original window is therefore the left-hand or upper of the two, and the new window is the right-hand or lower.

If window is omitted or nil, then the selected window is split. If size is omitted or nil, then window is divided evenly into two parts. (If there is an odd line, it is allocated to the new window.) When split-window is called interactively, all its arguments are nil.

The following example starts with one window on a screen that is 50 lines high by 80 columns wide; then the window is split.

(setq w (selected-window)) => #<window 8 on windows.texi> (window-edges) ; Edges in order: => (0 0 80 50) ; left--top--right--bottom ;; Returns window created (setq w2 (split-window w 15)) => #<window 28 on windows.texi> (window-edges w2) => (0 15 80 50) ; Bottom window; ; top is line 15 (window-edges w) => (0 0 80 15) ; Top window

The screen looks like this:

__________ | | line 0 | w | |__________| | | line 15 | w2 | |__________| line 50 column 0 column 80

Next, the top window is split horizontally:

(setq w3 (split-window w 35 t)) => #<window 32 on windows.texi> (window-edges w3) => (35 0 80 15) ; Left edge at column 35 (window-edges w) => (0 0 35 15) ; Right edge at column 35 (window-edges w2) => (0 15 80 50) ; Bottom window unchanged

Now, the screen looks like this:

column 35 __________ | | | line 0 | w | w3 | |___|______| | | line 15 | w2 | |__________| line 50 column 0 column 80

Normally, Emacs indicates the border between two side-by-side windows with a scroll bar (see section Scroll Bars) or `|' characters. The display table can specify alternative border characters; see 38.17 Display Tables.

Command: split-window-vertically &optional size

This function splits the selected window into two windows, one above the other, leaving the upper of the two windows selected, with size lines. (If size is negative, then the lower of the two windows gets - size lines and the upper window gets the rest, but the upper window is still the one selected.)

Command: split-window-horizontally &optional size

This function splits the selected window into two windows side-by-side, leaving the selected window with size columns.

This function is basically an interface to split-window. You could define a simplified version of the function like this:

(defun split-window-horizontally (&optional arg) "Split selected window into two windows, side by side..." (interactive "P") (let ((size (and arg (prefix-numeric-value arg)))) (and size (< size 0) (setq size (+ (window-width) size))) (split-window nil size t)))

Function: one-window-p &optional no-mini all-frames

This function returns non-nil if there is only one window. The argument no-mini, if non-nil, means don't count the minibuffer even if it is active; otherwise, the minibuffer window is included, if active, in the total number of windows, which is compared against one.

The argument all-frames specifies which frames to consider. Here are the possible values and their meanings:

nil

Count the windows in the selected frame, plus the minibuffer used by that frame even if it lies in some other frame.

t

Count all windows in all existing frames.

visible

Count all windows in all visible frames.

0

Count all windows in all visible or iconified frames.

anything else

Count precisely the windows in the selected frame, and no others.

28.3 Deleting Windows

A window remains visible on its frame unless you delete it by calling certain functions that delete windows. A deleted window cannot appear on the screen, but continues to exist as a Lisp object until there are no references to it. There is no way to cancel the deletion of a window aside from restoring a saved window configuration (see section 28.17 Window Configurations). Restoring a window configuration also deletes any windows that aren't part of that configuration.

When you delete a window, the space it took up is given to one adjacent sibling.

Function: window-live-p window

This function returns nil if window is deleted, and t otherwise.

Warning: Erroneous information or fatal errors may result from using a deleted window as if it were live.

Command: delete-window &optional window

This function removes window from display, and returns nil. If window is omitted, then the selected window is deleted. An error is signaled if there is only one window when delete-window is called.

Command: delete-other-windows &optional window

This function makes window the only window on its frame, by deleting the other windows in that frame. If window is omitted or nil, then the selected window is used by default.

The return value is nil.

Command: delete-windows-on buffer &optional frame

This function deletes all windows showing buffer. If there are no windows showing buffer, it does nothing.

delete-windows-on operates frame by frame. If a frame has several windows showing different buffers, then those showing buffer are removed, and the others expand to fill the space. If all windows in some frame are showing buffer (including the case where there is only one window), then the frame reverts to having a single window showing another buffer chosen with other-buffer. See section 27.8 The Buffer List.

The argument frame controls which frames to operate on. This function does not use it in quite the same way as the other functions which scan all windows; specifically, the values t and nil have the opposite of their meanings in other functions. Here are the full details:

• If it is nil, operate on all frames.
• If it is t, operate on the selected frame.
• If it is visible, operate on all visible frames.
• If it is 0, operate on all visible or iconified frames.
• If it is a frame, operate on that frame.

This function always returns nil.

28.4 Selecting Windows

When a window is selected, the buffer in the window becomes the current buffer, and the cursor will appear in it.

Function: selected-window

This function returns the selected window. This is the window in which the cursor appears and to which many commands apply.

Function: select-window window

This function makes window the selected window. The cursor then appears in window (on redisplay). The buffer being displayed in window is immediately designated the current buffer.

The return value is window.

(setq w (next-window)) (select-window w) => #<window 65 on windows.texi>

Macro: save-selected-window forms...

This macro records the selected window, executes forms in sequence, then restores the earlier selected window, unless it is no longer alive.

This macro does not save or restore anything about the sizes, arrangement or contents of windows; therefore, if the forms change them, the change persists.

Each frame, at any time, has a window selected within the frame. This macro saves only the selected window; it does not save anything about other frames. If the forms select some other frame and alter the window selected within it, the change persists.

The following functions choose one of the windows on the screen, offering various criteria for the choice.

Function: get-lru-window &optional frame

This function returns the window least recently "used" (that is, selected). The selected window is always the most recently used window.

The selected window can be the least recently used window if it is the only window. A newly created window becomes the least recently used window until it is selected. A minibuffer window is never a candidate.

The argument frame controls which windows are considered.

• If it is nil, consider windows on the selected frame.
• If it is t, consider windows on all frames.
• If it is visible, consider windows on all visible frames.
• If it is 0, consider windows on all visible or iconified frames.
• If it is a frame, consider windows on that frame.

Function: get-largest-window &optional frame

This function returns the window with the largest area (height times width). If there are no side-by-side windows, then this is the window with the most lines. A minibuffer window is never a candidate.

If there are two windows of the same size, then the function returns the window that is first in the cyclic ordering of windows (see following section), starting from the selected window.

The argument frame controls which set of windows to consider. See get-lru-window, above.

Function: get-window-with-predicate predicate &optional minibuf all-frames default

This function returns a window satisfying predicate. It cycles through all visible windows using walk-windows (see section 28.5 Cyclic Ordering of Windows), calling predicate on each one one of them with that window as its argument. The function returns the first window for which predicate returns a non-nil value; if that never happens, it returns default.

The optional arguments minibuf and all-frames specify the set of windows to include in the scan. See the description of next-window in 28.5 Cyclic Ordering of Windows, for details.

28.5 Cyclic Ordering of Windows

When you use the command C-x o (other-window) to select the next window, it moves through all the windows on the screen in a specific cyclic order. For any given configuration of windows, this order never varies. It is called the cyclic ordering of windows.

This ordering generally goes from top to bottom, and from left to right. But it may go down first or go right first, depending on the order in which the windows were split.

If the first split was vertical (into windows one above each other), and then the subwindows were split horizontally, then the ordering is left to right in the top of the frame, and then left to right in the next lower part of the frame, and so on. If the first split was horizontal, the ordering is top to bottom in the left part, and so on. In general, within each set of siblings at any level in the window tree, the order is left to right, or top to bottom.

Function: next-window &optional window minibuf all-frames

This function returns the window following window in the cyclic ordering of windows. This is the window that C-x o would select if typed when window is selected. If window is the only window visible, then this function returns window. If omitted, window defaults to the selected window.

The value of the argument minibuf determines whether the minibuffer is included in the window order. Normally, when minibuf is nil, the minibuffer is included if it is currently active; this is the behavior of C-x o. (The minibuffer window is active while the minibuffer is in use. See section 20. Minibuffers.)

If minibuf is t, then the cyclic ordering includes the minibuffer window even if it is not active.

If minibuf is neither t nor nil, then the minibuffer window is not included even if it is active.

The argument all-frames specifies which frames to consider. Here are the possible values and their meanings:

nil

Consider all the windows in window's frame, plus the minibuffer used by that frame even if it lies in some other frame.

t

Consider all windows in all existing frames.

visible

Consider all windows in all visible frames. (To get useful results, you must ensure window is in a visible frame.)

0

Consider all windows in all visible or iconified frames.

anything else

Consider precisely the windows in window's frame, and no others.

This example assumes there are two windows, both displaying the buffer `windows.texi':

(selected-window) => #<window 56 on windows.texi> (next-window (selected-window)) => #<window 52 on windows.texi> (next-window (next-window (selected-window))) => #<window 56 on windows.texi>

Function: previous-window &optional window minibuf all-frames

This function returns the window preceding window in the cyclic ordering of windows. The other arguments specify which windows to include in the cycle, as in next-window.

Command: other-window count &optional all-frames

This function selects the countth following window in the cyclic order. If count is negative, then it moves back -count windows in the cycle, rather than forward. It returns nil.

The argument all-frames has the same meaning as in next-window, but the minibuf argument of next-window is always effectively nil.

In an interactive call, count is the numeric prefix argument.

Function: walk-windows proc &optional minibuf all-frames

This function cycles through all windows, calling proc once for each window with the window as its sole argument.

The optional arguments minibuf and all-frames specify the set of windows to include in the scan. See next-window, above, for details.

Function: window-list &optional frame minibuf window

This function returns a list of the windows on frame, starting with window. If frame is nil or omitted, the selected frame is used instead; if window is nil or omitted, the selected window is used instead.

The value of minibuf determines if the minibuffer window will be included in the result list. If minibuf is t, the minibuffer window will be included, even if it isn't active. If minibuf is nil or omitted, the minibuffer window will only be included in the list if it is active. If minibuf is neither nil nor t, the minibuffer window is not included, whether or not it is active.

28.6 Buffers and Windows

This section describes low-level functions to examine windows or to display buffers in windows in a precisely controlled fashion. See section 28.7 Displaying Buffers in Windows, for related functions that find a window to use and specify a buffer for it. The functions described there are easier to use than these, but they employ heuristics in choosing or creating a window; use these functions when you need complete control.

Function: set-window-buffer window buffer-or-name

This function makes window display buffer-or-name as its contents. It returns nil. This is the fundamental primitive for changing which buffer is displayed in a window, and all ways of doing that call this function.

(set-window-buffer (selected-window) "foo") => nil

Function: window-buffer &optional window

This function returns the buffer that window is displaying. If window is omitted, this function returns the buffer for the selected window.

(window-buffer) => #<buffer windows.texi>

Function: get-buffer-window buffer-or-name &optional all-frames

This function returns a window currently displaying buffer-or-name, or nil if there is none. If there are several such windows, then the function returns the first one in the cyclic ordering of windows, starting from the selected window. See section 28.5 Cyclic Ordering of Windows.

The argument all-frames controls which windows to consider.

• If it is nil, consider windows on the selected frame.
• If it is t, consider windows on all frames.
• If it is visible, consider windows on all visible frames.
• If it is 0, consider windows on all visible or iconified frames.
• If it is a frame, consider windows on that frame.

Function: get-buffer-window-list buffer-or-name &optional minibuf all-frames

This function returns a list of all the windows currently displaying buffer-or-name.

The two optional arguments work like the optional arguments of next-window (see section 28.5 Cyclic Ordering of Windows); they are not like the single optional argument of get-buffer-window. Perhaps we should change get-buffer-window in the future to make it compatible with the other functions.

The argument all-frames controls which windows to consider.

• If it is nil, consider windows on the selected frame.
• If it is t, consider windows on all frames.
• If it is visible, consider windows on all visible frames.
• If it is 0, consider windows on all visible or iconified frames.
• If it is a frame, consider windows on that frame.

Variable: buffer-display-time

This variable records the time at which a buffer was last made visible in a window. It is always local in each buffer; each time set-window-buffer is called, it sets this variable to (current-time) in the specified buffer (see section 40.5 Time of Day). When a buffer is first created, buffer-display-time starts out with the value nil.

28.7 Displaying Buffers in Windows

In this section we describe convenient functions that choose a window automatically and use it to display a specified buffer. These functions can also split an existing window in certain circumstances. We also describe variables that parameterize the heuristics used for choosing a window. See section 28.6 Buffers and Windows, for low-level functions that give you more precise control. All of these functions work by calling set-window-buffer.

Do not use the functions in this section in order to make a buffer current so that a Lisp program can access or modify it; they are too drastic for that purpose, since they change the display of buffers in windows, which would be gratuitous and surprise the user. Instead, use set-buffer and save-current-buffer (see section 27.2 The Current Buffer), which designate buffers as current for programmed access without affecting the display of buffers in windows.

Command: switch-to-buffer buffer-or-name &optional norecord

This function makes buffer-or-name the current buffer, and also displays the buffer in the selected window. This means that a human can see the buffer and subsequent keyboard commands will apply to it. Contrast this with set-buffer, which makes buffer-or-name the current buffer but does not display it in the selected window. See section 27.2 The Current Buffer.

If buffer-or-name does not identify an existing buffer, then a new buffer by that name is created. The major mode for the new buffer is set according to the variable default-major-mode. See section 23.1.3 How Emacs Chooses a Major Mode.

Normally the specified buffer is put at the front of the buffer list (both the selected frame's buffer list and the frame-independent buffer list). This affects the operation of other-buffer. However, if norecord is non-nil, this is not done. See section 27.8 The Buffer List.

The switch-to-buffer function is often used interactively, as the binding of C-x b. It is also used frequently in programs. It always returns nil.

Command: switch-to-buffer-other-window buffer-or-name &optional norecord

This function makes buffer-or-name the current buffer and displays it in a window not currently selected. It then selects that window. The handling of the buffer is the same as in switch-to-buffer.

The currently selected window is absolutely never used to do the job. If it is the only window, then it is split to make a distinct window for this purpose. If the selected window is already displaying the buffer, then it continues to do so, but another window is nonetheless found to display it in as well.

This function updates the buffer list just like switch-to-buffer unless norecord is non-nil.

Function: pop-to-buffer buffer-or-name &optional other-window norecord

This function makes buffer-or-name the current buffer and switches to it in some window, preferably not the window previously selected. The "popped-to" window becomes the selected window within its frame.

If the variable pop-up-frames is non-nil, pop-to-buffer looks for a window in any visible frame already displaying the buffer; if there is one, it returns that window and makes it be selected within its frame. If there is none, it creates a new frame and displays the buffer in it.

If pop-up-frames is nil, then pop-to-buffer operates entirely within the selected frame. (If the selected frame has just a minibuffer, pop-to-buffer operates within the most recently selected frame that was not just a minibuffer.)

If the variable pop-up-windows is non-nil, windows may be split to create a new window that is different from the original window. For details, see 28.8 Choosing a Window for Display.

If other-window is non-nil, pop-to-buffer finds or creates another window even if buffer-or-name is already visible in the selected window. Thus buffer-or-name could end up displayed in two windows. On the other hand, if buffer-or-name is already displayed in the selected window and other-window is nil, then the selected window is considered sufficient display for buffer-or-name, so that nothing needs to be done.

All the variables that affect display-buffer affect pop-to-buffer as well. See section 28.8 Choosing a Window for Display.

If buffer-or-name is a string that does not name an existing buffer, a buffer by that name is created. The major mode for the new buffer is set according to the variable default-major-mode. See section 23.1.3 How Emacs Chooses a Major Mode.

This function updates the buffer list just like switch-to-buffer unless norecord is non-nil.

Command: replace-buffer-in-windows buffer

This function replaces buffer with some other buffer in all windows displaying it. The other buffer used is chosen with other-buffer. In the usual applications of this function, you don't care which other buffer is used; you just want to make sure that buffer is no longer displayed.

This function returns nil.

28.8 Choosing a Window for Display

This section describes the basic facility that chooses a window to display a buffer in---display-buffer. All the higher-level functions and commands use this subroutine. Here we describe how to use display-buffer and how to customize it.

Command: display-buffer buffer-or-name &optional not-this-window frame

This command makes buffer-or-name appear in some window, like pop-to-buffer, but it does not select that window and does not make the buffer current. The identity of the selected window is unaltered by this function.

If not-this-window is non-nil, it means to display the specified buffer in a window other than the selected one, even if it is already on display in the selected window. This can cause the buffer to appear in two windows at once. Otherwise, if buffer-or-name is already being displayed in any window, that is good enough, so this function does nothing.

display-buffer returns the window chosen to display buffer-or-name.

If the argument frame is non-nil, it specifies which frames to check when deciding whether the buffer is already displayed. If the buffer is already displayed in some window on one of these frames, display-buffer simply returns that window. Here are the possible values of frame:

• If it is nil, consider windows on the selected frame.
• If it is t, consider windows on all frames.
• If it is visible, consider windows on all visible frames.
• If it is 0, consider windows on all visible or iconified frames.
• If it is a frame, consider windows on that frame.

Precisely how display-buffer finds or creates a window depends on the variables described below.

User Option: display-buffer-reuse-frames

If this variable is non-nil, display-buffer searches existing frames for a window displaying the buffer. If the buffer is already displayed in a window in some frame, display-buffer makes the frame visible and raises it, to use that window. If the buffer is not already displayed, or if display-buffer-reuse-frames is nil, display-buffer's behavior is determined by other variables, described below.

User Option: pop-up-windows

This variable controls whether display-buffer makes new windows. If it is non-nil and there is only one window, then that window is split. If it is nil, then display-buffer does not split the single window, but uses it whole.

User Option: split-height-threshold

This variable determines when display-buffer may split a window, if there are multiple windows. display-buffer always splits the largest window if it has at least this many lines. If the largest window is not this tall, it is split only if it is the sole window and pop-up-windows is non-nil.

User Option: even-window-heights

This variable determines if display-buffer should even out window heights if the buffer gets displayed in an existing window, above or beneath another existing window. If even-window-heights is t, the default, window heights will be evened out. If even-window-heights is nil, the orginal window heights will be left alone.

User Option: pop-up-frames

This variable controls whether display-buffer makes new frames. If it is non-nil, display-buffer looks for an existing window already displaying the desired buffer, on any visible frame. If it finds one, it returns that window. Otherwise it makes a new frame. The variables pop-up-windows and split-height-threshold do not matter if pop-up-frames is non-nil.

If pop-up-frames is nil, then display-buffer either splits a window or reuses one.

See section 29. Frames, for more information.

Variable: pop-up-frame-function

This variable specifies how to make a new frame if pop-up-frames is non-nil.

Its value should be a function of no arguments. When display-buffer makes a new frame, it does so by calling that function, which should return a frame. The default value of the variable is a function that creates a frame using parameters from pop-up-frame-alist.

User Option: pop-up-frame-alist

This variable holds an alist specifying frame parameters used when display-buffer makes a new frame. See section 29.3 Frame Parameters, for more information about frame parameters.

User Option: special-display-buffer-names

A list of buffer names for buffers that should be displayed specially. If the buffer's name is in this list, display-buffer handles the buffer specially.

By default, special display means to give the buffer a dedicated frame.

If an element is a list, instead of a string, then the CAR of the list is the buffer name, and the rest of the list says how to create the frame. There are two possibilities for the rest of the list. It can be an alist, specifying frame parameters, or it can contain a function and arguments to give to it. (The function's first argument is always the buffer to be displayed; the arguments from the list come after that.)

User Option: special-display-regexps

A list of regular expressions that specify buffers that should be displayed specially. If the buffer's name matches any of the regular expressions in this list, display-buffer handles the buffer specially.

By default, special display means to give the buffer a dedicated frame.

If an element is a list, instead of a string, then the CAR of the list is the regular expression, and the rest of the list says how to create the frame. See above, under special-display-buffer-names.

Variable: special-display-function

This variable holds the function to call to display a buffer specially. It receives the buffer as an argument, and should return the window in which it is displayed.

The default value of this variable is special-display-popup-frame.

Function: special-display-popup-frame buffer &rest args

This function makes buffer visible in a frame of its own. If buffer is already displayed in a window in some frame, it makes the frame visible and raises it, to use that window. Otherwise, it creates a frame that will be dedicated to buffer.

If args is an alist, it specifies frame parameters for the new frame.

If args is a list whose CAR is a symbol, then (car args) is called as a function to actually create and set up the frame; it is called with buffer as first argument, and (cdr args) as additional arguments.

This function always uses an existing window displaying buffer, whether or not it is in a frame of its own; but if you set up the above variables in your init file, before buffer was created, then presumably the window was previously made by this function.

User Option: special-display-frame-alist

This variable holds frame parameters for special-display-popup-frame to use when it creates a frame.

User Option: same-window-buffer-names

A list of buffer names for buffers that should be displayed in the selected window. If the buffer's name is in this list, display-buffer handles the buffer by switching to it in the selected window.

User Option: 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, display-buffer handles the buffer by switching to it in the selected window.

Variable: display-buffer-function

This variable is the most flexible way to customize the behavior of display-buffer. If it is non-nil, it should be a function that display-buffer calls to do the work. The function should accept two arguments, the same two arguments that display-buffer received. It should choose or create a window, display the specified buffer, and then return the window.

This hook takes precedence over all the other options and hooks described above.

A window can be marked as "dedicated" to its buffer. Then display-buffer will not try to use that window to display any other buffer.

Function: window-dedicated-p window

This function returns t if window is marked as dedicated; otherwise nil.

Function: set-window-dedicated-p window flag

This function marks window as dedicated if flag is non-nil, and nondedicated otherwise.

28.9 Windows and Point

Each window has its own value of 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.

• 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.

• 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.

• 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.

• See section 30. Positions, for more details on buffer positions.

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.

Function: window-point &optional window

This function returns the current position of point in window. For a nonselected window, this is the value point would have (in that window's buffer) if that window were selected. If window is nil, the selected window is used.

When window is the selected window and its buffer is also the current buffer, the value returned is the same as point in that buffer.

Strictly speaking, it would be more correct to return the "top-level" value of point, outside of any save-excursion forms. But that value is hard to find.

Function: set-window-point window position

This function positions point in window at position position in window's buffer.

28.10 The Window Start Position

Each window contains a marker used to keep track of a buffer position that specifies where in the buffer display should start. This position is called the display-start position of the window (or just the 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.

Function: window-start &optional window

This function returns the display-start position of window window. If window is nil, the selected window is used. For example,

(window-start) => 7058

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 1 if the buffer doesn't have any.

Redisplay updates the window-start position (if you have not specified it explicitly since the previous redisplay)---for example, 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.

For a realistic example of using window-start, see the description of count-lines in 30.2.4 Motion by Text Lines.

Function: window-end &optional window update

This function returns the position of the end of the display in window window. If window is nil, the selected window is used.

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

If the last redisplay of 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 nil.

If update is non-nil, window-end always returns an up-to-date value for where the window ends, based on the current window-start value. If the saved value is valid, window-end returns that; otherwise it computes the correct value by scanning the buffer text.

Even if update is non-nil, 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 window-start value. In effect, it reports where the displayed text will end if scrolling is not required.

Function: set-window-start window position &optional noforce

This function sets the display-start position of window to position in window's buffer. It returns 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 nil for noforce, it means you want display to start at 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 is 1 and you set the start of the window to 2, then point would 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:

;; Here is what `foo' looks like before executing ;; the set-window-start expression. ---------- Buffer: foo ---------- -!-This is the contents of buffer foo. 2 3 4 5 6 ---------- Buffer: foo ---------- (set-window-start (selected-window) (1+ (window-start))) => 2 ;; Here is what `foo' looks like after executing ;; the set-window-start expression. ---------- Buffer: foo ---------- his is the contents of buffer foo. 2 3 -!-4 5 6 ---------- Buffer: foo ----------

If noforce is non-nil, and 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 position is not used.

Function: pos-visible-in-window-p &optional position window partially

This function returns t if position is within the range of text currently visible on the screen in window. It returns nil if position is scrolled vertically or horizontally out of view. Locations that are partially obscured are not considered visible unless partially is non-nil. The argument position defaults to the current position of point in window; window, to the selected window.

Here is an example:

(or (pos-visible-in-window-p (point) (selected-window)) (recenter 0))

28.11 Textual Scrolling

Textual scrolling means moving the text up or down though a window. It works by changing the value of the window's display-start location. It may also change the value of window-point to keep point on the screen.

Textual scrolling was formerly called "vertical scrolling," but we changed its name to distinguish it from the new vertical fractional scrolling feature (see section 28.12 Vertical Fractional Scrolling).

In the commands scroll-up and scroll-down, the directions "up" and "down" refer to the motion of the text in the buffer at which you are looking through 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 text in the middle of a buffer and repeatedly call scroll-down, you will eventually see the beginning of the buffer.

Some people have urged that the opposite convention be used: they imagine that the window moves over text that remains in place. Then "down" commands would take you to the end of the buffer. This view is more consistent with the actual relationship between windows and the text in the buffer, but it is less like what the user sees. The position of a window on the terminal does not move, and short scrolling commands clearly move the text up or down on the screen. We have chosen names that fit the user's point of view.

The textual scrolling functions (aside from scroll-other-window) have unpredictable results if the current buffer is different from the buffer that is displayed in the selected window. See section 27.2 The Current Buffer.

Command: scroll-up &optional count

This function scrolls the text in the selected window upward count lines. If count is negative, scrolling is actually downward.

If count is nil (or omitted), then the length of scroll is next-screen-context-lines lines less than the usable height of the window (not counting its mode line).

scroll-up returns nil.

Command: scroll-down &optional count

This function scrolls the text in the selected window downward count lines. If count is negative, scrolling is actually upward.

If count is omitted or nil, then the length of the scroll is next-screen-context-lines lines less than the usable height of the window (not counting its mode line).

scroll-down returns nil.

Command: scroll-other-window &optional count

This function scrolls the text in another window upward count lines. Negative values of count, or nil, are handled as in scroll-up.

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

When the selected window is the minibuffer, the next window is normally the one at the top left corner. You can specify a different window to scroll, when the minibuffer is selected, by setting the variable minibuffer-scroll-window. This variable has no effect when any other window is selected. See section 20.9 Minibuffer Miscellany.

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, 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 "Beginning of buffer".

Variable: other-window-scroll-buffer

If this variable is non-nil, it tells scroll-other-window which buffer to scroll.

User Option: 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, the window scrolls automatically (if possible) to move point out of the margin, closer to the center of the window.

User Option: 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 zero, then redisplay scrolls the text to center point vertically in the window. If the value is a positive integer n, then redisplay scrolls the window up to n lines in either direction, if that will bring point back into view. Otherwise, it centers point. The default value is zero.

User Option: scroll-down-aggressively

The value of this variable should be either nil or a fraction 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 f part of the window height from the top. The larger f, the more aggressive the scrolling.

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

User Option: scroll-up-aggressively

Likewise, for scrolling up. The value, f, specifies how far point should be placed from the bottom of the window; thus, as with scroll-up-aggressively, a larger value scrolls more aggressively.

User Option: scroll-step

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

User Option: scroll-preserve-screen-position

If this option is non-nil, the scroll functions move point so that the vertical position of the cursor is unchanged, when that is possible.

User Option: 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, scroll-up with an argument of nil scrolls so that this many lines at the bottom of the window appear instead at the top. The default value is 2.

Command: recenter &optional count

This function scrolls the selected window to put the text where point is located at a specified vertical position within the window.

If count is a nonnegative number, it puts the line containing point count lines down from the top of the window. If count is a negative number, then it counts upward from the bottom of the window, so that -1 stands for the last usable line in the window. If count is a non-nil list, then it stands for the line in the middle of the window.

If count is nil, recenter puts the line containing point in the middle of the window, then clears and redisplays the entire selected frame.

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

With an argument of zero, recenter positions the current line at the top of the window. This action is so handy that some people make a separate key binding to do this. For example,

(defun line-to-top-of-window () "Scroll current line to top of window. Replaces three keystroke sequence C-u 0 C-l." (interactive) (recenter 0)) (global-set-key [kp-multiply] 'line-to-top-of-window)

28.12 Vertical Fractional Scrolling

Vertical fractional scrolling means shifting the image in the window up or down by a specified multiple or fraction of a line. Starting in Emacs 21, each window has a 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.

Function: window-vscroll &optional window

This function returns the current vertical scroll position of window, If window is nil, the selected window is used.

(window-vscroll) => 0

Function: set-window-vscroll window lines

This function sets window's vertical scroll position to lines. The argument 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.

(set-window-vscroll (selected-window) 1.2) => 1.13

28.13 Horizontal Scrolling

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 vertical 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 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.

In Emacs 21, redisplay automatically alters the horizontal scrolling of a window as necessary to ensure that point is always visible, if automatic-hscrolling is set. 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.

Command: scroll-left &optional count

This function scrolls the selected window count columns to the left (or to the right if count is negative). The default for 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 window-hscroll (below).

Command: scroll-right &optional count

This function scrolls the selected window count columns to the right (or to the left if count is negative). The default for 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 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.

Function: window-hscroll &optional window

This function returns the total leftward horizontal scrolling of window---the number of columns by which the text in window is scrolled left past the left margin.

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

If window is nil, the selected window is used.

(window-hscroll) => 0 (scroll-left 5) => 5 (window-hscroll) => 5

Function: set-window-hscroll window columns

This function sets the number of columns from the left margin that window is scrolled from the value of columns. The argument columns should be zero or positive; if not, it is taken as zero. Fractional values of columns are not supported at present.

The value returned is columns.

(set-window-hscroll (selected-window) 10) => 10

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

(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)))))

28.14 The Size of a Window

An Emacs window is rectangular, and its size information consists of the height (the number of lines) and the width (the number of character positions in each line). The mode line is included in the height. But the width does not count the scroll bar or the column of `|' characters that separates side-by-side windows.

The following three functions return size information about a window:

Function: window-height &optional window

This function returns the number of lines in window, including its mode line. If window fills its entire frame, this is typically one less than the value of frame-height on that frame (since the last line is always reserved for the minibuffer).

If window is nil, the function uses the selected window.

(window-height) => 23 (split-window-vertically) => #<window 4 on windows.texi> (window-height) => 11

Function: window-width &optional window

This function returns the number of columns in window. If window fills its entire frame, this is the same as the value of frame-width on that frame. The width does not include the window's scroll bar or the column of `|' characters that separates side-by-side windows.

If window is nil, the function uses the selected window.

(window-width) => 80

Function: window-edges &optional window

This function returns a list of the edge coordinates of window. If window is nil, the selected window is used.

The order of the list is (left top right bottom), all elements relative to 0, 0 at the top left corner of the frame. The element right of the value is one more than the rightmost column used by window, and bottom is one more than the bottommost row used by window and its mode-line.

If a window has a scroll bar, the right edge value includes the width of the scroll bar. Otherwise, if the window has a neighbor on the right, its right edge value includes the width of the separator line between the window and that neighbor. Since the width of the window does not include this separator, the width does not usually equal the difference between the right and left edges.

Here is the result obtained on a typical 24-line terminal with just one window:

(window-edges (selected-window)) => (0 0 80 23)

The bottom edge is at line 23 because the last line is the echo area.

If window is at the upper left corner of its frame, then bottom is the same as the value of (window-height), right is almost the same as the value of (window-width), and top and left are zero. For example, the edges of the following window are `0 0 8 5'. Assuming that the frame has more than 8 columns, the last column of the window (column 7) holds a border rather than text. The last row (row 4) holds the mode line, shown here with `xxxxxxxxx'.

0 _______ 0 | | | | | | | | xxxxxxxxx 4 7

In the following example, let's suppose that the frame is 7 columns wide. Then the edges of the left window are `0 0 4 3' and the edges of the right window are `4 0 8 3'.

___ ___ | | | | | | xxxxxxxxx 0 34 7

28.15 Changing the Size of a Window

The window size functions fall into two classes: high-level commands that change the size of windows and low-level functions that access window size. Emacs does not permit overlapping windows or gaps between windows, so resizing one window affects other windows.

Command: enlarge-window size &optional horizontal

This function makes the selected window size lines taller, stealing lines from neighboring windows. It takes the lines from one window at a time until that window is used up, then takes from another. If a window from which lines are stolen shrinks below window-min-height lines, that window disappears.

If horizontal is non-nil, this function makes window wider by size columns, stealing columns instead of lines. If a window from which columns are stolen shrinks below window-min-width columns, that window disappears.

If the requested size would exceed that of the window's frame, then the function makes the window occupy the entire height (or width) of the frame.

If there are various other windows from which lines or columns can be stolen, and some of them specify fixed size (using window-size-fixed, see below), they are left untouched while other windows are "robbed." If it would be necessary to alter the size of a fixed-size window, enlarge-window gets an error instead.

If size is negative, this function shrinks the window by -size lines or columns. If that makes the window smaller than the minimum size (window-min-height and window-min-width), enlarge-window deletes the window.

enlarge-window returns nil.

Command: enlarge-window-horizontally columns

This function makes the selected window columns wider. It could be defined as follows:

(defun enlarge-window-horizontally (columns) (enlarge-window columns t))

Command: shrink-window size &optional horizontal

This function is like enlarge-window but negates the argument size, making the selected window smaller by giving lines (or columns) to the other windows. If the window shrinks below window-min-height or window-min-width, then it disappears.

If size is negative, the window is enlarged by -size lines or columns.

Command: shrink-window-horizontally columns

This function makes the selected window columns narrower. It could be defined as follows:

(defun shrink-window-horizontally (columns) (shrink-window columns t))

Command: shrink-window-if-larger-than-buffer &optional window

This command shrinks window to be as small as possible while still showing the full contents of its buffer--but not less than window-min-height lines. If window is not given, it defaults to the selected window.

However, the command does nothing if the window is already too small to display the whole text of the buffer, or if part of the contents are currently scrolled off screen, or if the window is not the full width of its frame, or if the window is the only window in its frame.

Variable: window-size-fixed

If this variable is non-nil, in any given buffer, then the size of any window displaying the buffer remains fixed unless you explicitly change it or Emacs has no other choice. (This feature is new in Emacs 21.)

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

The usual way to use this variable is to give it a buffer-local value in a particular buffer. That way, the windows (but usually there is only one) displaying that buffer have fixed size.

Explicit size-change functions such as enlarge-window get an error if they would have to change a window size which is fixed. Therefore, when you want to change the size of such a window, you should bind window-size-fixed to nil, like this:

(let ((window-size-fixed nil)) (enlarge-window 10))

Note that changing the frame size will change the size of a fixed-size window, if there is no other alternative.

The following two variables constrain the window-size-changing functions to a minimum height and width.

User Option: window-min-height

The value of this variable determines how short a window may become before it is automatically deleted. Making a window smaller than window-min-height automatically deletes it, and no window may be created shorter than this. The absolute minimum height is two (allowing one line for the mode line, and one line for the buffer display). Actions that change window sizes reset this variable to two if it is less than two. The default value is 4.

User Option: window-min-width

The value of this variable determines how narrow a window may become before it is automatically deleted. Making a window smaller than window-min-width automatically deletes it, and no window may be created narrower than this. The absolute minimum width is one; any value below that is ignored. The default value is 10.

28.16 Coordinates and Windows

This section describes how to relate screen coordinates to windows.

Function: window-at x y &optional frame

This function returns the window containing the specified cursor position in the frame frame. The coordinates x and y are measured in characters and count from the top left corner of the frame. If they are out of range, window-at returns nil.

If you omit frame, the selected frame is used.

Function: coordinates-in-window-p coordinates window

This function checks whether a particular frame position falls within the window window.

The argument coordinates is a cons cell of the form (x . y). The coordinates x and y are measured in characters, and count from the top left corner of the screen or frame.

The value returned by coordinates-in-window-p is non-nil if the coordinates are inside window. The value also indicates what part of the window the position is in, as follows:

(relx . rely)

The coordinates are inside window. The numbers relx and rely are the equivalent window-relative coordinates for the specified position, counting from 0 at the top left corner of the window.

mode-line

The coordinates are in the mode line of window.

header-line

The coordinates are in the header line of window.

vertical-line

The coordinates are in the vertical line between 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.

nil

The coordinates are not in any part of window.

The function coordinates-in-window-p does not require a frame as argument because it always uses the frame that window is on.

28.17 Window Configurations

A window configuration records the entire layout of one frame--all windows, their sizes, which buffers they contain, what part of each buffer is displayed, and the values of point and the mark. You can bring back an entire previous layout by restoring a window configuration previously saved.

If you want to record all frames instead of just one, use a frame configuration instead of a window configuration. See section 29.12 Frame Configurations.

Function: current-window-configuration &optional frame

This function returns a new object representing frame's current window configuration, including the number of windows, their sizes and current buffers, which window is the selected window, and for each window the displayed buffer, the display-start position, and the positions of point and the mark. It also includes the values of window-min-height, window-min-width and minibuffer-scroll-window. An exception is made for point in the current buffer, whose value is not saved.

If frame is omitted, the selected frame is used.

Function: set-window-configuration configuration

This function restores the configuration of windows and buffers as specified by configuration, for the frame that configuration was created for.

The argument configuration must be a value that was previously returned by current-window-configuration. This configuration is restored in the frame from which configuration was made, whether that frame is selected or not. This always counts as a window size change and triggers execution of the window-size-change-functions (see section 28.18 Hooks for Window Scrolling and Changes), because set-window-configuration doesn't know how to tell whether the new configuration actually differs from the old one.

If the frame which configuration was saved from is dead, all this function does is restore the three variables window-min-height, window-min-width and minibuffer-scroll-window.

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

(let ((config (current-window-configuration))) (unwind-protect (progn (split-window-vertically nil) ...) (set-window-configuration config)))

Special Form: save-window-excursion forms...

This special form records the window configuration, executes forms in sequence, then restores the earlier window configuration. The window configuration includes the value of point and the portion of the buffer that is visible. It also includes the choice of selected window. However, it does not include the value of point in the current buffer; use save-excursion also, if you wish to preserve that.

Don't use this construct when save-selected-window is all you need.

Exit from save-window-excursion always triggers execution of the window-size-change-functions. (It doesn't know how to tell whether the restored configuration actually differs from the one in effect at the end of the forms.)

The return value is the value of the final form in forms. For example:

(split-window) => #<window 25 on control.texi> (setq w (selected-window)) => #<window 19 on control.texi> (save-window-excursion (delete-other-windows w) (switch-to-buffer "foo") 'do-something) => do-something ;; The screen is now split again.

Function: window-configuration-p object

This function returns t if object is a window configuration.

Function: 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 t even if those aspects differ.

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

Primitives to look inside of window configurations would make sense, but none are implemented. It is not clear they are useful enough to be worth implementing.

28.18 Hooks for Window Scrolling and Changes

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 window-scroll-functions; the last runs window-size-change-functions. The paradigmatic use of these hooks is in the implementation of Lazy Lock mode; see section `Font Lock Support Modes' in The GNU Emacs Manual.

Variable: window-scroll-functions

This variable holds a list of functions that Emacs should call before redisplaying a window with scrolling. It is not a normal hook, because each function is called with two arguments: the window, and its new display-start position.

Displaying a different buffer in the window also runs these functions.

These functions must be careful in using window-end (see section 28.10 The Window Start Position); if you need an up-to-date value, you must use the update argument to ensure you get it.

Variable: 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.

It is not a good idea to use save-window-excursion (see section 28.17 Window Configurations) in these functions, because that always counts as a size change, and it would cause these functions to be called over and over. In most cases, save-selected-window (see section 28.4 Selecting Windows) is what you need here.

Variable: redisplay-end-trigger-functions

This abnormal hook is run whenever redisplay in a window uses text that extends past a specified end trigger position. You set the end trigger position with the function set-window-redisplay-end-trigger. The functions are called with two arguments: the window, and the end trigger position. Storing nil for the end trigger position turns off the feature, and the trigger value is automatically reset to nil just after the hook is run.

Function: set-window-redisplay-end-trigger window position

This function sets window's end trigger position at position.

Function: window-redisplay-end-trigger &optional window

This function returns window's current end trigger position.

Variable: 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 frame whose window configuration has changed is the selected frame when this hook runs.