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* Documenting buffer display
@ 2018-10-20 12:20 martin rudalics
  2018-10-20 13:21 ` Eli Zaretskii
                   ` (2 more replies)
  0 siblings, 3 replies; 32+ messages in thread
From: martin rudalics @ 2018-10-20 12:20 UTC (permalink / raw)
  To: emacs-devel

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Attached find a rewrite of the documentation of buffer display
functions to be applied against the release branch.

Work in progress.  Proofreading and suggestions welcome.

Thanks for the attention, martin

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diff --git a/doc/emacs/windows.texi b/doc/emacs/windows.texi
index 7dbd680..7ad9a77 100644
--- a/doc/emacs/windows.texi
+++ b/doc/emacs/windows.texi
@@ -48,8 +48,8 @@ Basic Window
 @kbd{C-x 4 b} that select a different window and switch buffers in it.
 Also, all commands that display information in a window, including
 (for example) @kbd{C-h f} (@code{describe-function}) and @kbd{C-x C-b}
-(@code{list-buffers}), work by switching buffers in a nonselected
-window without affecting the selected window.
+(@code{list-buffers}), usually work by displaying buffers in a
+nonselected window without affecting the selected window.

   When multiple windows show the same buffer, they can have different
 regions, because they can have different values of point.  However,
@@ -340,11 +340,9 @@ Displaying Buffers
 in response to a user command.  There are several different ways in
 which commands do this.

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

   Some commands try to display intelligently, trying not to take
 over the selected window, e.g., by splitting off a new window and
@@ -367,10 +365,9 @@ Displaying Buffers

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

 @menu
 * Window Choice::   How @code{display-buffer} works.
@@ -388,28 +385,56 @@ Window Choice
 sequence of steps.

 @itemize
-@vindex same-window-buffer-names
-@vindex same-window-regexps
 @item
 First, check if the buffer should be displayed in the selected window
 regardless of other considerations.  You can tell Emacs to do this by
-adding the desired buffer's name to the list
-@code{same-window-buffer-names}, or adding a matching regular
-expression to the list @code{same-window-regexps}.  By default, these
-variables are @code{nil}, so this step is skipped.
+adding a regular expression matching the buffer's name together with a
+reference to the @code{display-buffer-same-window} action function
+(@pxref{Action Functions,,Action Functions for Buffer Display, elisp,
+The Emacs Lisp Reference Manual}) to the option
+@code{display-buffer-alist} (@pxref{Choosing Window,,Choosing a Window
+for Display, elisp, The Emacs Lisp Reference Manual}).  For example,
+to display the buffer @file{*scratch*} preferably in the selected
+window write:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ '("\\*scratch\\*" (display-buffer-same-window)))
+@end group
+@end example
+
+By default, @code{display-buffer-alist} is @code{nil}, so this step is
+skipped.

 @item
 Otherwise, if the buffer is already displayed in an existing window,
-reuse that window.  Normally, only windows on the selected frame
-are considered, but windows on other frames are also reusable if you
-change @code{pop-up-frames} (see below) to @code{t}.
+reuse that window.  Normally, only windows on the selected frame are
+considered, but windows on other frames are also reusable if a
+corresponding @code{reusable-frames} action alist entry (@pxref{Action
+Alists,,Action Alists for Buffer Display, elisp, The Emacs Lisp
+Reference Manual}) is used (see the next step for an example of how to
+do that).

-@vindex pop-up-frames
 @item
 Otherwise, optionally create a new frame and display the buffer there.
-By default, this step is skipped.  To enable it, change the variable
-@code{pop-up-frames} to a non-@code{nil} value.  The special value
-@code{graphic-only} means to do this only on graphical displays.
+By default, this step is skipped.  To enable it, change the value of
+the option @code{display-buffer-base-action} (@pxref{Choosing
+Window,,Choosing a Window for Display, elisp, The Emacs Lisp Reference
+Manual}) as follows:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-base-action
+ '((display-buffer-reuse-window display-buffer-pop-up-frame)
+   (reusable-frames . 0)))
+@end group
+@end example
+
+This customization will also try to make the preceding step search for
+a reusable window on all visible of iconified frames

 @item
 Otherwise, try to create a new window by splitting a window on the
@@ -429,9 +454,9 @@ Window Choice

 @item
 Otherwise, display the buffer in a window previously showing it.
-Normally, only windows on the selected frame are considered, but if
-@code{pop-up-frames} is non-@code{nil} the window may be also on another
-frame.
+Normally, only windows on the selected frame are considered, but with
+a suitable @code{reusable-frames} action alist entry (see above) the
+window may be also on another frame.

 @item
 Otherwise, display the buffer in an existing window on the selected
@@ -442,14 +467,9 @@ Window Choice
 and display the buffer there.
 @end itemize

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

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

 Some buffers are shown in windows for perusal rather than for editing.
@@ -459,24 +479,23 @@ Temporary Displays
 displayed only for a short period of time.

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

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

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

diff --git a/doc/lispref/elisp.texi b/doc/lispref/elisp.texi
index 6c3182b..e0939e5 100644
--- a/doc/lispref/elisp.texi
+++ b/doc/lispref/elisp.texi
@@ -1043,9 +1043,7 @@ Top
 * 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.
+* Displaying Buffers::      Displaying a buffer in a suitable window.
 * Window History::          Each window remembers the buffers displayed in it.
 * Dedicated Windows::       How to avoid displaying another buffer in
                               a specific window.
@@ -1067,6 +1065,18 @@ Top
                               redisplay going past a certain point,
                               or window configuration changes.

+Displaying Buffers
+
+* Choosing Window::         How to choose a window for displaying a buffer.
+* Action Functions::        Support functions for buffer display.
+* Action Alists::           Alists for fine-tuning buffer display
+                              action functions.
+* Choosing Window Options:: Extra options affecting how buffers are displayed.
+* Precedence of Action Functions:: A tutorial explaining the precedence of
+                              buffer display action functions.
+* The Zen of Buffer Display:: How to avoid that buffers get lost in between
+                              windows.
+
 Side Windows

 * Displaying Buffers in Side Windows:: An action function for displaying
diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
index 1e008da..31fb7ce 100644
--- a/doc/lispref/frames.texi
+++ b/doc/lispref/frames.texi
@@ -3198,11 +3198,10 @@ Child Frames
 @code{drag-with-mode-line} parameter.

   When a child frame is used for displaying a buffer via
-@code{display-buffer-in-child-frame} (@pxref{Display Action Functions}),
-the frame's @code{auto-hide-function} parameter (@pxref{Frame
-Interaction Parameters}) can be set to a function, in order to
-appropriately deal with the frame when the window displaying the buffer
-shall be quit.
+@code{display-buffer-in-child-frame} (@pxref{Action Functions}), the
+frame's @code{auto-hide-function} parameter (@pxref{Frame Interaction
+Parameters}) can be set to a function, in order to appropriately deal
+with the frame when the window displaying the buffer shall be quit.

   When a child frame is used during minibuffer interaction, for example,
 to display completions in a separate window, the @code{minibuffer-exit}
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 2650671..c14c441 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -25,9 +25,7 @@ Windows
 * 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.
+* Displaying Buffers::      Displaying a buffer in a suitable window.
 * Window History::          Each window remembers the buffers displayed in it.
 * Dedicated Windows::       How to avoid displaying another buffer in
                               a specific window.
@@ -162,6 +160,7 @@ Basic Windows
 in the group).
 @end defun

+
 @node Windows and Frames
 @section Windows and Frames

@@ -1542,11 +1541,11 @@ Recombining Windows
 window is created anyway).

 @item window-size
-This means that @code{display-buffer} makes a new parent window when it
-splits a window and is passed a @code{window-height} or
-@code{window-width} entry in the @var{alist} argument (@pxref{Display
-Action Functions}).  Otherwise, window splitting behaves as for a value
-of @code{nil}.
+This means that @code{display-buffer} makes a new parent window when
+it splits a window and is passed a @code{window-height} or
+@code{window-width} entry in the @var{alist} argument (@pxref{Action
+Functions}).  Otherwise, window splitting behaves as for a value of
+@code{nil}.

 @item temp-buffer-resize
 In this case @code{with-temp-buffer-window} makes a new parent window
@@ -1879,7 +1878,7 @@ Cyclic Window Ordering
 @cindex ordering of windows, cyclic
 @cindex window ordering, cyclic

-  When you use the command @kbd{C-x o} (@code{other-window}) to select
+  When you use the command @w{@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}.
@@ -1899,7 +1898,7 @@ Cyclic Window Ordering
 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
+active; this matches the behavior of @w{@kbd{C-x o}}.  (Note that a
 minibuffer window is active as long as its minibuffer is in use; see
 @ref{Minibuffers}).

@@ -2327,10 +2326,39 @@ Switching Buffers
 @end deffn


+@node Displaying Buffers
+@section Displaying a Buffer in a Suitable Window
+
+In this section we describe how Emacs finds or creates a window
+suitable for displaying a buffer.  We first introduce the function
+@code{display-buffer}---the workhorse for choosing such a window.
+Next, action functions---auxiliary functions called by
+@code{display-buffer} to find or create a suitable window---are
+presented.  Then we describe action alists---special association lists
+used to fine-tune the behavior of action functions.
+
+   We continue with the description of additional options to customize
+the behavior of @code{display-buffer}.  Then a series of examples will
+try to explain the precedence among action functions in a single call
+of @code{display-buffer}.  We conclude this section with some
+guidelines for managing the complexity of buffer display.
+
+@menu
+* Choosing Window::         How to choose a window for displaying a buffer.
+* Action Functions::        Support functions for buffer display.
+* Action Alists::           Alists for fine-tuning buffer display.
+* Choosing Window Options:: Extra options affecting how buffers are displayed.
+* Precedence of Action Functions:: Examples to explain the precedence of
+                              action functions.
+* The Zen of Buffer Display:: How to avoid that buffers get lost in between
+                              windows.
+@end menu
+
+
 @node Choosing Window
-@section Choosing a Window for Display
+@subsection Choosing a Window for Displaying a Buffer

-  The command @code{display-buffer} flexibly chooses a window for
+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
@@ -2351,8 +2379,9 @@ Choosing Window
 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.
+@xref{Action Functions}, for a list of predefined action functions.
+@xref{Action Alists}, for a list of action alist entries recognized by
+these functions.

   @code{display-buffer} works by combining display actions from
 several sources, and calling the action functions in turn, until one
@@ -2364,11 +2393,13 @@ Choosing Window
 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.
+It is @code{nil} if no suitable window was found.

 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):
+actions from the following sources (in order of their precedence,
+highest ranking first):

 @itemize
 @item
@@ -2388,12 +2419,35 @@ Choosing Window
 @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 caller can
-pass @code{(allow-no-window . t)} as an element of the action alist to
-indicate its readiness to handle the case of not displaying the
-buffer in a window.
+In practice this means that @code{display-buffer} builds a list of all
+action functions specified by these display actions.  The first
+element of this list is the first action function specified by
+@code{display-buffer-overriding-action}, if any.  The last element of
+this list is @code{display-buffer-pop-up-frame}---the last action
+function specified by @code{display-buffer-fallback-action}.
+Duplicates are not removed from this list---hence the same action
+function may be called multiple times during one call of
+@code{display-buffer}.
+
+@code{display-buffer} calls the action functions specified by this
+list 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}.
+
+Note that the second argument is always the list of @emph{all} action
+alist entries specified by the sources named above.  Hence, the first
+element of this list is the first action alist entry specified by
+@code{display-buffer-overriding-action}, if any.  The last element of
+this list is the last alist entry specified by
+@code{display-buffer-base-action}, if any (the action alist specified
+by @code{display-buffer-fallback-action} is empty).
+
+Note also, that the combined action alist may contain duplicate
+entries and entries for the same key with different values.  As a
+rule, action functions always use the first association for each key
+so users may not expect that the association found by the action
+function is the one provided by the same display action,
+see @ref{Precedence of Action Functions} for an example.

 The argument @var{action} can also have a non-@code{nil}, non-list
 value.  This has the special meaning that the buffer should be
@@ -2404,8 +2458,8 @@ Choosing Window
 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}.
+. @var{frame})} to the action alist of @var{action} (@pxref{Action
+Alists}).
 @end deffn

 @defvar display-buffer-overriding-action
@@ -2437,13 +2491,16 @@ Choosing Window
 @end defvr


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

 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.
+display, and @var{alist}, an action alist.  As a rule, each action
+function is supposed to return a window displaying @var{buffer} if it
+succeeds and @code{nil} if it fails (for the sole exception to this
+rule see the last function listed
+below---@code{display-buffer-no-window}).

 @defun display-buffer-same-window buffer alist
 This function tries to display @var{buffer} in the selected window.
@@ -2453,57 +2510,103 @@ Display Action Functions
 @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.
+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
-
-Note that these meanings differ slightly from those of the
-@var{all-frames} argument to @code{next-window} (@pxref{Cyclic Window
-Ordering}).
-
-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}.
+the selected window is not eligible for reuse.  The set of frames to
+search for a window already displaying @var{buffer} can be specified
+with the help of a @code{reusable-frames} action alist entry
+(@pxref{Action Alists}).  If @var{alist} contains no
+@code{reusable-frames} entry, this function searches just the selected
+frame.

-If this function chooses a window on another frame, it makes that frame
-visible and, unless @var{alist} contains an @code{inhibit-switch-frame}
-entry (@pxref{Choosing Window Options}), raises that frame if necessary.
+If this function chooses a window on another frame, it makes that
+frame visible and, unless @var{alist} contains an
+@code{inhibit-switch-frame} entry (@pxref{Action Alists}), raises that
+frame if necessary.
 @end defun

 @defun display-buffer-reuse-mode-window buffer alist
 This function tries to display @var{buffer} by finding a window
 that is displaying a buffer in a given mode.

-If @var{alist} contains a @code{mode} entry, its value is a major mode
-(a symbol) or a list of major modes.  If @var{alist} contains no
-@code{mode} entry, the current major mode of @var{buffer} is used.  A
-window is a candidate if it displays a buffer that derives from one of
-the given modes.
+If @var{alist} contains a @code{mode} entry, its value specifes a
+major mode (a symbol) or a list of major modes.  If @var{alist}
+contains no @code{mode} entry, the current major mode of @var{buffer}
+is used instead.  A window is a candidate if it displays a buffer
+whose mode derives from one of the modes specified thusly.

-The behavior is also controlled by entries for
+The behavior is also controlled by alist entries for
 @code{inhibit-same-window}, @code{reusable-frames} and
-@code{inhibit-switch-frame} as is done in the function
-@code{display-buffer-reuse-window}.
+@code{inhibit-switch-frame} (@pxref{Action Alists}) as is done in the
+function @code{display-buffer-reuse-window}.
+@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 by
+@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}
+(@pxref{Action Alists}).  If @var{alist} contains a
+@code{preserve-size} entry (@pxref{Action Alists}), Emacs will try to
+preserve the size of the new window during future resize operations
+(@pxref{Preserving Window Sizes}).
+
+This function fails if no window can be split.  More often than not
+this happens because no window is large enough to allow splitting.
+Setting @code{split-height-threshold} or @code{split-width-threshold}
+(@pxref{Choosing Window Options}) to a lower value may help in this
+regard.  It also happens when the selected frame has an
+@code{unsplittable} frame parameter; @pxref{Buffer Parameters}.
+@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
+(@pxref{Action Alists}).
+
+If @var{alist} has a @code{previous-window} entry and the window
+specified by that entry is live and not dedicated to another buffer,
+that window will be preferred, even if it never showed @var{buffer}
+before.
+@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.  If there is a window below the selected one and that
+window already displays @var{buffer}, it reuses that window.
+
+If there is no such window, this function tries to create a new window
+by splitting the selected one and display @var{buffer} there.  It will
+also adjust that window's size provided @var{alist} contains a suitable
+@code{window-height} or @code{window-width} entry, see above.
+
+If splitting the selected window fails and there is a non-dedicated
+window below the selected one showing some other buffer, it uses that
+window for showing @var{buffer}.
+@end defun
+
+@defun display-buffer-at-bottom buffer alist
+This function tries to display @var{buffer} in a window at the bottom
+of the selected frame.
+
+This either splits the window at the bottom of the frame or the
+frame's root window, or reuses an existing window at the bottom of the
+selected frame.
 @end defun

 @defun display-buffer-pop-up-frame buffer alist
@@ -2537,11 +2640,12 @@ Display Action Functions
 @defun display-buffer-use-some-frame buffer alist
 This function tries to display @var{buffer} by trying to find a
 frame that meets a predicate (by default any frame other than the
-current frame).
+selected frame).

-If this function chooses a window on another frame, it makes that frame
-visible and, unless @var{alist} contains an @code{inhibit-switch-frame}
-entry (@pxref{Choosing Window Options}), raises that frame if necessary.
+If this function chooses a window on another frame, it makes that
+frame visible and, unless @var{alist} contains an
+@code{inhibit-switch-frame} entry (@pxref{Action Alists}), raises that
+frame if necessary.

 If @var{alist} has a non-@code{nil} @code{frame-predicate} entry, its
 value is a function taking one argument (a frame), returning
@@ -2549,207 +2653,220 @@ Display Action Functions
 default predicate.

 If @var{alist} has a non-@code{nil} @code{inhibit-same-window} entry,
-the selected window is used; thus if the selected frame has a single
-window, it is not used.
+the selected window is not used; thus if the selected frame has a
+single window, it is not used.
 @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}).
+@defun display-buffer-no-window buffer alist
+If @var{alist} has a non-@code{nil} @code{allow-no-window} entry, then
+this function does not display @code{buffer} and returns the symbol
+@code{fail}.  This will skip the excution of any further display
+actions and cause @code{display-buffer} to return @code{nil}.

-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:
+   It is assumed that when a call of @code{display-buffer} specifies a
+non-@code{nil} @code{allow-no-window} entry, the caller can handle a
+@code{nil} return value.
+@end defun
+
+If the @var{alist} argument of any of these functions contains a
+@code{window-parameters} entry, @code{display-buffer} assigns the
+elements of the associated value as window parameters of the chosen
+window.
+
+
+@node Action Alists
+@subsection Action Alists for Buffer Display
+
+An action alist (@pxref{Choosing Window}) is an association list
+mapping predefined symbols recognized by action functions to values
+these functions are supposed to interpret accordingly.  In each call,
+@code{display-buffer} builds a new, possibly empty, action alist and
+passes that entire list on to the action functions it calls.
+
+   By design, action functions are free in their interpretation of
+action alist entries.  In fact, some entries like
+@code{allow-no-window} or @code{previous-window} have a meaning only
+for one or a few action functions and are ignored by the rest.  Other
+entries, like @code{inhibit-same-window} or @code{window-parameters},
+are supposed to be respected by most action functions including those
+provided by application programs and external packages.
+
+   In the previous subsection we have described in detail how
+individual action functions interpret the action alist entries they
+care about.  Here we give a reference list of all known action alist
+entries according to their symbols, together with their values and
+action functions that recognize them.  Throughout this list, the terms
+``buffer'' will refer to the buffer @code{display-buffer} is supposed
+to display and ``value'' to the entry's value.
+
+@table @code
+@item allow-no-window
+If the value is non-@code{nil}, @code{display-buffer} does not
+necessarily have to display the buffer and the caller is prepared to
+accept that.  This entry is not intended for user customizations since
+there is no guarantee that an arbitrary caller of
+@code{display-buffer} will be able to handle the case that no window
+will display the buffer.  @code{display-buffer-no-window} is the only
+action function that cares about this entry.
+
+@item inhibit-same-window
+If the value is non-@code{nil}, this signals that the selected window
+must not be used for displaying the buffer.  All action functions that
+(re-)use an existing window respect this entry.
+
+@item previous-window
+The value must specify a window that may have displayed the buffer
+previously.  @code{display-buffer-in-previous-window} will give
+preference to such a window provided it is still live and not
+dedicated to another buffer.
+
+@item mode
+The value is either a major mode or a list of major modes.
+@code{display-buffer-reuse-mode-window} may reuse a window whenever
+the value specified by this entry matches the major mode of that
+window's buffer.  Other action functions ignore such entries.
+
+@item frame-predicate
+The value must be a function taking one argument (a frame), supposed
+to return non-@code{nil} if that frame is a candidate for displaying
+the buffer.  This entry is used by
+@code{display-buffer-use-some-frame}.
+
+@item pop-up-frame-parameters
+The value specifies an alist of frame parameters to give a new frame,
+if one is created.  @code{display-buffer-pop-up-frame} is its one and
+only one addressee.
+
+@item reusable-frames
+The value specifies the frame(s) to search for a window that can be
+reused because it already displays the buffer.  It can be set as
+follows:
+
+@itemize @bullet
+@item
+@code{nil} means consider only windows on the selected frame.
+(Actually, the last frame used that is not a minibuffer-only 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
+
+Note that the meaning of @code{nil} differs slightly from that of the
+@var{all-frames} argument to @code{next-window} (@pxref{Cyclic Window
+Ordering}).
+
+A major client of this is @code{display-buffer-reuse-window} but all
+other action functions that try to reuse a window are affected as
+well.
+
+@item inhibit-switch-frame
+A non-@code{nil} value prevents another frame from being raised or
+selected, if the window chosen by @code{display-buffer} is displayed
+there.  Primarily affected by this are
+@code{display-buffer-use-some-frame} and
+@code{display-buffer-reuse-window}.
+@code{display-buffer-pop-up-frame} should be affected as well but
+there is no guarantee that the window manager will not override it.
+
+@item window-parameters
+The value specifies an alist of window parameters to give the chosen
+window.  All action functions that choose a window should process this
+entry.
+
+@item window-height
+The value specifies whether and how to adjust the height of the chosen
+window and can be provided as follows:

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

 @item
-A number specifies the desired height of the new window.  An integer
-specifies the number of lines of the window.  A floating-point
+A number specifies the desired height of the chosen window.  An
+integer 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
+If the value specifies a function, that function is called with one
+argument---the chosen 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:
+All action functions that choose a window should process this entry.
+
+@item window-width
+This entry is similar to the @code{window-height} entry described
+before but can be used to adjust the chosen window's width instead.
+The value can be one of the following:

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

 @item
-A number specifies the desired width of the new window.  An integer
+A number specifies the desired width of the chosen window.  An integer
 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.
+If the value specifies a function, that function is called with one
+argument---the chosen window.  The function is supposed to adjust the
+width of the window; its return value is ignored.
 @end itemize

-If @var{alist} contains a @code{preserve-size} entry, Emacs will try to
-preserve the size of the new window during future resize operations
-(@pxref{Preserving Window Sizes}).  The @sc{cdr} of that entry must be a
-cons cell whose @sc{car}, if non-@code{nil}, means to preserve the width
-of the window and whose @sc{cdr}, if non-@code{nil}, means to preserve
-the height of the window.
-
-This function can fail if no window splitting can be performed for some
-reason (e.g., if the selected frame has an @code{unsplittable} frame
-parameter; @pxref{Buffer Parameters}).
-@end defun
-
-@defun display-buffer-below-selected buffer alist
-This function tries to display @var{buffer} in a window below the
-selected window.  If there is a window below the selected one and that
-window already displays @var{buffer}, it reuses that window.
-
-If there is no such window, this function tries to create a new window
-by splitting the selected one and display @var{buffer} there.  It will
-also adjust that window's size provided @var{alist} contains a suitable
-@code{window-height} or @code{window-width} entry, see above.
-
-If splitting the selected window fails and there is a non-dedicated
-window below the selected one showing some other buffer, it uses that
-window for showing @var{buffer}.
-@end defun
+All action functions that choose a window should process this entry.

-@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}.
+@item preserve-size
+If non-@code{nil} such an entry tells Emacs to preserve the size of
+the window chosen (@pxref{Preserving Window Sizes}).  The value should
+be either @code{(t . nil)} to preserve the width of the window,
+@code{(nil . t)} to preserve its height or @code{(t . t)} to preserve
+both, its width and its height.  All action functions that choose a
+window should process this entry.

-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
+@item child-frame-parameters
+The value specifies an alist of frame parameters used when the buffer
+is displayed on a child frame.  This entry is used by
+@code{display-buffer-in-child-frame} only.

-@defun display-buffer-at-bottom buffer alist
-This function tries to display @var{buffer} in a window at the bottom
-of the selected frame.
-
-This either splits the window at the bottom of the frame or the
-frame's root window, or reuses an existing window at the bottom of the
-selected frame.
-@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-no-window buffer alist
-If @var{alist} has a non-@code{nil} @code{allow-no-window} entry, then
-this function does not display @code{buffer}.  This allows you to
-override the default action and avoid displaying the buffer.  It is
-assumed that when the caller specifies a non-@code{nil}
-@code{allow-no-window} value it can handle a @code{nil} value returned
-from @code{display-buffer} in this case.
-@end defun
-
-If the @var{alist} argument of any of these functions contains a
-@code{window-parameters} entry, @code{display-buffer} assigns the
-elements of the associated value as window parameters of the chosen
-window.
-
-   To illustrate the use of action functions, consider the following
-example.
-
-@example
-@group
-(display-buffer
- (get-buffer-create "*foo*")
- '((display-buffer-reuse-window
-    display-buffer-pop-up-window
-    display-buffer-pop-up-frame)
-   (reusable-frames . 0)
-   (window-height . 10) (window-width . 40)))
-@end group
-@end example
-
-@noindent
-Evaluating the form above will cause @code{display-buffer} to proceed as
-follows: If a buffer called *foo* already appears on a visible or
-iconified frame, it will reuse its window.  Otherwise, it will try to
-pop up a new window or, if that is impossible, a new frame and show the
-buffer there.  If all these steps fail, it will proceed using whatever
-@code{display-buffer-base-action} and
-@code{display-buffer-fallback-action} prescribe.
-
-   Furthermore, @code{display-buffer} will try to adjust a reused window
-(provided *foo* was put by @code{display-buffer} there before) or a
-popped-up window as follows: If the window is part of a vertical
-combination, it will set its height to ten lines.  Note that if, instead
-of the number 10, we specified the function
-@code{fit-window-to-buffer}, @code{display-buffer} would come up with a
-one-line window to fit the empty buffer.  If the window is part of a
-horizontal combination, it sets its width to 40 columns.  Whether a new
-window is vertically or horizontally combined depends on the shape of
-the window split and the values of
-@code{split-window-preferred-function}, @code{split-height-threshold}
-and @code{split-width-threshold} (@pxref{Choosing Window Options}).
-
-   Now suppose we combine this call with a preexisting setup for
-@code{display-buffer-alist} as follows.
-
-@example
-@group
-(let ((display-buffer-alist
-       (cons
-        '("\\*foo\\*"
-          (display-buffer-reuse-window display-buffer-below-selected)
-          (reusable-frames)
-          (window-height . 5))
-        display-buffer-alist)))
-  (display-buffer
-   (get-buffer-create "*foo*")
-   '((display-buffer-reuse-window
-      display-buffer-pop-up-window
-      display-buffer-pop-up-frame)
-     (reusable-frames . 0)
-     (window-height . 10) (window-width . 40))))
-@end group
-@end example
+@item side
+The value denotes the side of the frame or window where a new window
+displaying the buffer shall be created.  This entry is used by
+@code{display-buffer-in-side-window} to indicate the side of the frame
+where a new side window shall be placed (@pxref{Displaying Buffers in
+Side Windows}).  It is also used by
+@code{display-buffer-in-atom-window} to indicate the side of an
+existing window where the new window shall be located (@pxref{Atomic
+Windows}).

-@noindent
-This form will have @code{display-buffer} first try reusing a window
-that shows *foo* on the selected frame.  If there's no such window, it
-will try to split the selected window or, if that is impossible, use the
-window below the selected window.
+@item slot
+If non-@code{nil}, the value specifies the slot of the side window
+supposed to display the buffer.  This entry is used by
+@code{display-buffer-in-side-window} only (@pxref{Displaying Buffers
+in Side Windows}).

-   If there's no window below the selected one, or the window below the
-selected one is dedicated to its buffer, @code{display-buffer} will
-proceed as described in the previous example.  Note, however, that when
-it tries to adjust the height of any reused or popped-up window, it will
-in any case try to set its number of lines to 5 since that value
-overrides the corresponding specification in the @var{action} argument
-of @code{display-buffer}.
+@item window
+The value specifies a window that is in some way related to the window
+chosen by @code{display-buffer}.  This entry is currently used by
+@code{display-buffer-in-atom-window} to indicate the window on whose
+side the new window shall be created (@pxref{Atomic Windows}).
+@end table


 @node Choosing Window Options
-@section Additional Options for Displaying Buffers
+@subsection 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
+The behavior of the display actions of @code{display-buffer}
+(@pxref{Choosing Window}) can be modified by the following user
 options.

 @defopt pop-up-windows
@@ -2757,10 +2874,10 @@ Choosing Window Options
 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
+This variable is provided for backward compatibility only.  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
+function @code{display-buffer-pop-up-window} (@pxref{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.
@@ -2770,7 +2887,7 @@ Choosing Window Options
 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 window (@pxref{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,
@@ -2797,19 +2914,19 @@ Choosing Window Options
 @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
+This variable specifies whether @code{split-window-sensibly} is
+allowed 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.
+This variable specifies whether @code{split-window-sensibly} is
+allowed 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 even-window-sizes
@@ -2827,30 +2944,11 @@ Choosing Window Options
 of their combination.
 @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}).
+@code{display-buffer-pop-up-frame} action function (@pxref{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
@@ -2860,30 +2958,632 @@ Choosing Window Options

 @defopt pop-up-frame-alist
 This variable holds an alist of frame parameters (@pxref{Frame
-Parameters}), which is used by the default function in
+Parameters}), which is used by the function specified by
 @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 showing it in the selected
-window.
+This option is provided for backward compatibility only.  Note,
+however that when @code{display-buffer-pop-up-frame} calls the
+function specified by @code{pop-up-frame-function}, it prepends the
+value of all @code{pop-up-frame-parameters} action alist entries to
+@code{pop-up-frame-alist} so that the values specified by the action
+alist entry effectively override the corresponding values of
+@code{pop-up-frame-alist}.
+
+Hence, users should set up a @code{pop-up-frame-parameters} action
+alist entry in @code{display-buffer-alist} instead of customizing
+@code{pop-up-frame-alist}.  Only this will guarantee that the value of
+a parameter specified by the user overrides the value of that
+parameter specified by the caller of @code{display-buffer}.
 @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 showing it in the selected window.
-@end defopt
+   Many efforts in the design of @code{display-buffer} have been given
+to maintain compatibility with code that uses older options like
+@code{pop-up-windows}, @code{pop-up-frames},
+@code{pop-up-frame-alist}, @code{same-window-buffer-names} and
+@code{same-window-regexps}.  Applications and users should refrain
+from using these options in future code.  Above we already warned
+against customizing @code{pop-up-frame-alist}.  Here we describe how
+to convert the remaining options to use display actions instead.
+
+@table @code
+@item pop-up-windows
+This variable is @code{nil} by default.  Instead of customizing it to
+@code{nil} and thus telling @code{display-buffer} what not to do, it's
+much better to list in @code{display-buffer-base-action} the actions
+@code{display-buffer} should do instead as, for example:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-base-action
+ '((display-buffer-reuse-window display-buffer-pop-up-frame
+    display-buffer-in-previous-window display-buffer-use-some-window)))
+@end group
+@end example
+
+@item pop-up-frames
+Instead of customizing this variable to @code{t}, customize
+@code{display-buffer-base-action}, for example, as follows:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-base-action
+ '((display-buffer-reuse-window display-buffer-pop-up-frame)
+   (reusable-frames . 0)))
+@end group
+@end example
+
+@item same-window-buffer-names
+@itemx same-window-regexps
+Instead of adding a buffer name or a regular expression to one of
+these options use a @code{display-buffer-alist} entry for that buffer
+specifying the action function @code{display-buffer-same-window}.
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ (cons '("\\*foo\\*" (display-buffer-same-window)) display-buffer-alist))
+@end group
+@end example
+@end table
+
+
+@node Precedence of Action Functions
+@subsection Precedence of Action Functions
+
+From the past subsections we already know that @code{display-buffer}
+must be supplied with a number of display actions (@pxref{Choosing
+Window}) in order to display a buffer.  In a completely uncustomized
+Emacs, these actions are specified by
+@code{display-buffer-fallback-action} in the following order of
+precedence: Reuse a window, pop up a new window on the same frame, use
+a window previously showing the buffer, use some window and pop up a
+new frame.  (The remaining actions in the default value of
+@code{display-buffer-fallback-action} are void in an uncustomized
+Emacs).
+
+Let's now consider the following form:
+
+@example
+(display-buffer (get-buffer-create "*foo*"))
+@end example
+
+@noindent
+Put this form into the buffer @file{*scratch*} of an uncustomized
+Emacs session you just started and evaluate it: @code{display-buffer}
+will fail to reuse a window showing @file{*foo*} but succeed in
+popping up a new window.  If you evaluate the form again, nothing will
+change---@code{display-buffer} reused the window already showing
+@file{*foo*} because that action was applicable and had the highest
+precedence among all applicable actions.
+
+   Popping up a new window will fail if there is not enough space on
+the selected frame.  In an uncustomized Emacs it typically fails when
+there are already two windows on a frame.  For example, if you now
+type @w{@kbd{C-x 1}} followed by @w{@kbd{C-x 2}} and evaluate the form
+once more, @file{*foo*} should show up in the lower
+window---@code{display-buffer} just used ``some'' window.  If, before
+typing @w{@kbd{C-x 2}} you had typed @w{@kbd{C-x o}}, @file{*foo*}
+would have been shown in the upper window because ``some'' window
+stands for the ``least recently used'' window and the selected window
+has been least recently used if and only if it is alone on its frame.
+
+   Let's assume you did not type @w{@kbd{C-x o}} and @file{*foo*} is
+shown in the lower window.  Type @w{@kbd{C-x o}} to get there followed
+by @w{@kbd{C-x left}} and evaluate the form again.  This should
+display @file{*foo*} in the same, lower window because that window had
+already shown @file{*foo*} previously and was therefore chosen instead
+of some other window (remember the precedence order prescribed by
+@code{display-buffer-fallback-action} we sketched above).
+
+  So far we have only observed default behaviors in an uncustomized
+Emacs session.  Next let's see how these behaviors can be customized.
+The first option we consider is @code{display-buffer-base-action}.  It
+provides a very coarse customization which conceptually affects the
+display of @emph{any} buffer.  It can be used to supplement the
+actions supplied by @code{display-buffer-fallback-action} by
+reordering them or by adding actions that are not present there but
+fit more closely the user's editing practice.  It can be, however,
+also used to change the default behavior in a more profound way.
+
+   Let's consider a user who, as a rule, prefers to display buffers on
+another frame.  Such a user might provide the following customization:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-base-action
+ '((display-buffer-reuse-window display-buffer-pop-up-frame)
+   (reusable-frames . 0)))
+@end group
+@end example
+
+@noindent
+This setting will cause@code{display-buffer} to first try to find a
+window showing the buffer on a visible or iconified frame and, if no
+such frame exists, pop up a new frame.  You can observe this behavior
+on a graphical system by typing @w{@kbd{C-x 1}} in the window showing
+@file{*scratch*} and evaluating the canonical @code{display-buffer}
+form.  This will usually create (and give focus to) a new frame whose
+root window shows @file{*foo*}.  Iconify that frame and evaluate the
+canonical form again: @code{display-buffer} will reuse the window on
+the new frame (usually raising the frame and giving it focus too).
+
+   Only if creating a new frame fails, @code{display-buffer} will
+apply the actions supplied by @code{display-buffer-fallback-action}
+which means to again try to reuse a window, pop up a new window and so
+on.  A trivial way to make frame creation fail is supplied by the
+following form:
+
+@example
+@group
+(let ((pop-up-frame-function 'ignore))
+  (display-buffer (get-buffer-create "*foo*")))
+@end group
+@end example
+
+@noindent
+We will forget about that form immediately after observing that it
+fails to create a new frame and uses a fallback action instead.
+
+   Note that @code{display-buffer-reuse-window} appears redundant in
+the customization of @code{display-buffer-base-action} because it is
+already part of @code{display-buffer-fallback-action} and should be
+tried there anyway.  However, that would fail because due to the
+precedence of @code{display-buffer-base-action} over
+@code{display-buffer-fallback-action}, at that time
+@code{display-buffer-pop-up-frame} would have already won the race.
+In fact
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-base-action
+ '(display-buffer-pop-up-frame (reusable-frames . 0)))
+@end group
+@end example
+
+@noindent
+would cause @code{display-buffer} to @emph{always} pop up a new frame
+which is probably not what our user wants.
+
+   So far, we have only shown how @emph{users} can customize the
+default behavior of @code{display-buffer}.  Let us now see how
+@emph{applications} can change the course of @code{display-buffer}.
+The canonical way to do that is to use the @var{action} argument of
+@code{display-buffer} or the function that eventually ends up calling
+it.
+
+   Suppose an application wants to display @file{*foo*} preferably
+below the selected window (to immediately attract the attention of the
+user to the new window) or, if that fails, in a window at the bottom
+of the frame.  It could do that with a call like
+
+@example
+@group
+(display-buffer
+ (get-buffer-create "*foo*")
+ '((display-buffer-below-selected display-buffer-at-bottom)))
+@end group
+@end example
+
+@noindent
+In order to see how this new, modified form works, delete any frame
+showing @file{*foo*}, type @w{@kbd{C-x 1}} followed by @w{@kbd{C-x 2}} in the
+window showing @file{*scratch*} and subsequently evaluate that form.
+@code{display-buffer} should split the upper window and show
+@file{*foo*} in the new window.  Alternatively, if after @w{@kbd{C-x 2}}
+you had typed @w{@kbd{C-x o}}, @code{display-buffer} would have split the
+window at the bottom instead.
+
+   Suppose now that, before evaluating the new form, you have made the
+selected window as small as possible, for example, by evaluating the
+form @code{(fit-window-to-buffer)} in that window.  In that case,
+@code{display-buffer} would have failed to split the selected window
+and would have split the frame's root window instead, effectively
+displaying @file{*foo*} at the bottom of the frame.
+
+   In either case, evaluating the new form a second time now should
+reuse the window already showing @file{*foo*} since both functions
+supplied by the @var{action} argument try to reuse such a window
+first.
+
+   By setting the @var{action} argument, an application effectively
+overrules any customization of @code{display-buffer-base-action}.  Our
+user can now either accept the choice of the application or redouble
+by customizing the option @code{display-buffer-alist} as follows:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ '(("\\*foo\\*"
+    (display-buffer-reuse-window display-buffer-pop-up-frame))))
+@end group
+@end example
+
+@noindent
+Trying this with the new, modified form above in a configuration that
+does not show @file{*foo*} anywhere, will display @file{*foo*} on a
+separate frame, completely ignoring the @var{action} argument of
+@code{display-buffer}.
+
+   Note that we didn't care to specify a @code{reusable-frames} action
+alist entry in our specification of @code{display-buffer-alist}.
+@code{display-buffer} always takes the first one it finds---in our
+case the one specified by @code{display-buffer-base-action}.  If we
+wanted to use a different specification, for example, to exclude
+iconified frames showing @file{*foo*} from the list of reusable ones,
+we would have to specify that separately, however:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ '(("\\*foo\\*"
+    (display-buffer-reuse-window display-buffer-pop-up-frame)
+    (reusable-frames . visible))))
+@end group
+@end example
+
+@noindent
+If you try this, you will notice that repeated attempts to display
+@file{*foo*} will succeed to reuse a frame only if that frame is
+visible.
+
+   The above example would allow the conclusion that users customize
+@code{display-buffer-alist} for the sole purpose to overrule the
+@var{action} argument chosen by applications.  Such a conclusion would
+be incorrect.  @code{display-buffer-alist} is the standard option for
+users to direct the course of display of specific buffers in a
+preferred way regardless of whether the display is also guided by an
+@var{action} argument.
+
+   We can, however, reasonably conclude that customizing
+@code{display-buffer-alist} differs from customizing
+@code{display-buffer-base-action} in two major aspects: It is stronger
+because it overrides the @var{action} argument of
+@code{display-buffer}.  And it allows to explicitly specify all
+affected buffers.  In fact displaying other buffers is not affected in
+any way by a customization for @file{*foo*}.  For example,
+
+@example
+(display-buffer (get-buffer-create "*bar*"))
+@end example
+
+@noindent
+continues being governed by the settings of
+@code{display-buffer-base-action} and
+@code{display-buffer-fallback-action} only.
+
+   We could stop with our examples here but applications still have an
+ace up their sleeves which they can use to overrule any customization
+of @code{display-buffer-alist}.  It's the variable
+@code{display-buffer-overriding-action} which they can bind around
+@code{display-buffer} calls as follows:
+
+@example
+@group
+(let ((display-buffer-overriding-action '((display-buffer-same-window))))
+  (display-buffer
+   (get-buffer-create "*foo*")
+   '((display-buffer-below-selected display-buffer-at-bottom))))
+@end group
+@end example
+
+@noindent
+Evaluating this form will usually display @file{*foo*} in the selected
+window regardless of the @var{action} argument and any user
+customizations.  It might be illustrative to look at the list of
+action functions @code{display-buffer} would have tried to display
+@file{*foo*} with the customizations we provided earlier.  The list
+(including comments explaining who added this and the subsequent
+elements) is:
+
+@example
+@group
+(display-buffer-same-window  ;; `display-buffer-overriding-action'
+ display-buffer-reuse-window ;; `display-buffer-alist'
+ display-buffer-pop-up-frame
+ display-buffer-below-selected ;; ACTION argument
+ display-buffer-at-bottom
+ display-buffer-reuse-window ;; `display-buffer-base-action'
+ display-buffer-pop-up-frame
+ display-buffer--maybe-same-window ;; `display-buffer-fallback-action'
+ display-buffer-reuse-window
+ display-buffer--maybe-pop-up-frame-or-window
+ display-buffer-in-previous-window
+ display-buffer-use-some-window
+ display-buffer-pop-up-frame)
+@end group
+@end example
+
+@noindent
+Among the internal functions listed here
+@code{display-buffer--maybe-same-window} is effectively ignored while
+@code{display-buffer--maybe-pop-up-frame-or-window} actually runs
+@code{display-buffer-pop-up-window}.
+
+The action alist passed in each function call is:
+
+@example
+@group
+((reusable-frames . visible)
+ (reusable-frames . 0))
+@end group
+@end example
+
+@noindent
+which shows that we have used the second specification of
+@code{display-buffer-alist} above.  Suppose our user had written that
+as
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ '(("\\*foo\\*"
+    (display-buffer-reuse-window display-buffer-pop-up-frame)
+    (inhibit-same-window . t)
+    (reusable-frames . visible))))
+@end group
+@end example
+
+@noindent
+In this case the @code{inhibit-same-window} alist entry will
+successfully invalidate the @code{display-buffer-same-window}
+specification from @code{display-buffer-overriding-action} and
+@code{display-buffer} will show @file{*foo*} on another frame.
+
+   This last example shows that while the precedence order of action
+functions is fixed as described in @ref{Choosing Window}, an action
+alist entry specified by a display action ranked lower in that order
+can affect the execution of an higher ranked display action.
+
+
+@node The Zen of Buffer Display
+@subsection The Zen of Buffer Display
+
+In its most simplistic form, a frame accommodates always one single
+window that can be used for displaying a buffer.  As a consequence, it
+is always the latest call of @code{display-buffer} that will have
+succeeded in placing its buffer there.
+
+   Since working with such a frame is not very practical, Emacs by
+default allows for more complex frame layouts controlled by the
+default values of the frame size and the @code{split-height-threshold}
+and @code{split-width-threshold} options.  Displaying a buffer not yet
+shown on a frame then either splits the single window on that frame or
+(re-)uses one of its two windows.
+
+   The default behavior is abandoned as soon as the user customizes
+one of these thresholds or manually changes the frame's layout.  The
+default behavior is also abandoned when calling @code{display-buffer}
+with a non-@code{nil} @var{action} argument or the user customizes one
+of the options mentioned in the previous subsections.  Mastering
+@code{display-buffer} soon may become a frustrating experience due to
+the plethora of applicable display actions and the resulting frame
+layouts.
+
+   However, refraining from using buffer display functions and falling
+back on a split & delete windows metaphor is not a good idea either.
+Buffer display functions give applications and users a framework to
+reconcile their different needs; no comparable framework exists for
+splitting and deleting windows.  They also allow to at least partially
+restore the layout of a frame when removing a buffer from it later
+(@pxref{Quitting Windows}).
+
+   Below we will give a number of guidelines to redeem the frustration
+mentioned above and thus to avoid that buffers literally get lost in
+between the windows of a frame.
+
+@table @asis
+@item Write display actions without stress
+Writing display actions can be a pain because one has to lump together
+action functions and action alists in one huge list.  (Historical
+reasons prevented us from having @code{display-buffer} support
+separate arguments for these.)  It might help to memorize some basic
+forms like the ones listed below:
+
+@example
+'(nil (inhibit-same-window . t))
+@end example
+
+@noindent
+specifies an action alist entry only and no action function.  Its sole
+purpose is to inhibit a @code{display-buffer-same-window} function
+specified elsewhere from showing the buffer in the same window, see
+also the last example of the preceding subsection.
+
+@example
+'(display-buffer-below-selected)
+@end example
+
+on the other hand specifies one action function and an empty action
+alist.  To combine the effects of the above two specifications one
+would write the form
+
+@example
+'(display-buffer-below-selected (inhibit-same-window . t))
+@end example
+
+to add another action function one would write
+
+@example
+@group
+'((display-buffer-below-selected display-buffer-at-bottom)
+  (inhibit-same-window . t))
+@end group
+@end example
+
+and to add another alist entry one would write
+
+@example
+@group
+'((display-buffer-below-selected display-buffer-at-bottom)
+  (inhibit-same-window . t) (window-height . fit-window-to-buffer))
+@end group
+@end example
+
+That last form can be used as @var{action} argument of
+@code{display-buffer} in the following way:
+
+@example
+@group
+(display-buffer
+ (get-buffer-create "*foo*")
+ '((display-buffer-below-selected display-buffer-at-bottom)
+   (inhibit-same-window . t) (window-height . fit-window-to-buffer)))
+@end group
+@end example
+
+In a customization of @code{display-buffer-alist} it would be used as
+follows:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ '(("\\*foo\\*"
+    (display-buffer-below-selected display-buffer-at-bottom)
+    (inhibit-same-window . t) (window-height . fit-window-to-buffer))))
+@end group
+@end example
+
+To add a customization for a second buffer one would then write:
+
+@example
+@group
+(customize-set-variable
+ 'display-buffer-alist
+ '(("\\*foo\\*"
+    (display-buffer-below-selected display-buffer-at-bottom)
+    (inhibit-same-window . t) (window-height . fit-window-to-buffer))
+   ("\\*bar\\*"
+    (display-buffer-reuse-window display-buffer-pop-up-frame)
+    (reusable-frames . visible))))
+@end group
+@end example
+
+@item Treat each other with respect
+@code{display-buffer-alist} and @code{display-buffer-base-action} are
+user options---applications must never set or rebind them.
+@code{display-buffer-overriding-action}, on the other hand, is
+reserved for applications---who seldom use that option and if they use
+it, then with utmost care.
+
+   Older implementations of @code{display-buffer} frequently caused
+users and applications to fight over the settings of user options like
+@code{pop-up-frames} and @code{pop-up-windows} (@pxref{Choosing Window
+Options}).  This was one major reason for redesigning
+@code{display-buffer}---to provide a clear rule dividing what users
+and applications should be allowed to do.
+
+   Applications must be prepared that a user's customizations may
+cause buffers to get displayed in an unexpected way.  They should
+never assume in their subsequent behavior, that the buffer has been
+shown precisely the way they asked for in the @var{action} argument of
+@code{display-buffer}.
+
+   Users should not pose too many and too severe restrictions on how
+arbitrary buffers gets displayed.  Otherwise, they will risk to lose
+the characteristics of showing a buffer for a certain purpose.
+Suppose an application has been written to compare different versions
+of a buffer in two windows side-by-side.  If the customization of
+@code{display-buffer-alist} prescribes that any such buffer should be
+always shown in or below the selected window, the application will
+have a hard time to set up the desired window configuration via
+@code{display-buffer}.
+
+   To specify a preference for showing an arbitrary buffer, users
+should customize @code{display-buffer-base-action}.  An example of how
+users who prefer working with multiple frames would do that was given
+in the previous subsection.  @code{display-buffer-alist} should be
+reserved for displaying specific buffers in a specific way.
+
+@item Consider reusing a window that already shows the buffer
+Generally, it's always a good idea for users and application
+programmers to be prepared for the case that a window already shows
+the buffer in question and to reuse that window.  In the preceding
+subsection we have shown that failing to do so properly may cause
+@code{display-buffer} to continuously pop up a new frame although a
+frame showing that buffer existed already.  Only sometimes, it might
+be undesirable to reuse a window, for example, when a different
+portion of the buffer should be shown in that window.
+
+   Hence, @code{display-buffer-reuse-window} is one action function
+that should be used as often as possible, both in @var{action}
+arguments and customizations.  An @code{inhibit-same-window} entry in
+the @var{action} argument usually takes care of the most common case
+where reusing a window showing the buffer should be avoided---that
+where the window in question is the selected one.
+
+@item Attract focus to the window chosen
+This is a no-brainer for people working with multiple frames---the
+frame showing the buffer will automatically raise and get focus unless
+an @code{inhibit-switch-frame} entry forbids it.  For single frame
+users this task can be considerably more difficult.  In particular,
+@code{display-buffer-pop-up-window} and
+@code{display-buffer-use-some-window} can become obtrusive in this
+regard.  They split or use a seemingly arbitrary (often the largest or
+least recently used) window, distracting the user's attention.
+
+Some applications therefore try to choose a window at the bottom of
+the frame, for example, in order to display the buffer in vicinity of
+the minibuffer window where the user is expected to answer a question
+related to the new window.  For non-input related actions
+@code{display-buffer-below-selected} might be preferable because the
+selected window usually already has the user's attention.
+
+@item Handle subsequent invocations of @code{display-buffer}
+@code{display-buffer} is not overly well suited for displaying several
+buffers in sequence and making sure that all these buffers are shown
+orderly in the resulting window configuration.  Again, the standard
+action functions @code{display-buffer-pop-up-window} and
+@code{display-buffer-use-some-window} are not very suited for this
+purpose due to their somewhat chaotic nature in more complex
+configurations.
+
+   To produce a window configuration displaying multiple buffers (or
+different views of one and the same buffer) in one and the same
+display cycle, application programmers will unavoidably have to write
+their own action functions.  A few tricks listed below might help in
+this regard.
+
+@itemize @bullet
+@item
+Making windows atomic (@pxref{Atomic Windows}) avoids that an
+existing window composition gets broken when popping up a new window.
+The new window will pop up outside the composition instead.
+
+@item
+Temporarily dedicating windows to their buffers (@pxref{Dedicated
+Windows}) avoids that a window gets used for displaying a different
+buffer.  A non-dedicated window will be used instead.
+
+@item
+Calling @code{window-preserve-size} (@pxref{Preserving Window Sizes})
+will try to keep the size of the argument window unchanged when
+popping up a new window.  You have to make sure that another window in
+the same combination can be shrunk instead, though.
+
+@item
+Side windows (@pxref{Side Windows}) can be used for displaying
+specific buffers always in a window at the same position of a frame.
+This permits to group buffers that do not compete for being shown at
+the same time on a frame and show any such buffer in the same window
+without disrupting the display of other buffers.
+
+@item
+Child frames (@pxref{Child Frames}) can be used to display a buffer
+within the screen estate of the selected frame without disrupting that
+frame's window configuration and without the overhead associated with
+full-fledged frames as inflicted by @code{display-buffer-pop-up-frame}.
+@end itemize
+@end table

-@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
@@ -3204,7 +3904,7 @@ Side Windows
    In their most simple form of use, side windows allow to display
 specific buffers always in the same area of a frame.  Hence they can be
 regarded as a generalization of the concept provided by
-@code{display-buffer-at-bottom} (@pxref{Display Action Functions}) to
+@code{display-buffer-at-bottom} (@pxref{Action Functions}) to
 the remaining sides of a frame.  With suitable customizations, however,
 side windows can be also used to provide frame layouts similar to those
 found in so-called integrated development environments (IDEs).
@@ -3221,8 +3921,8 @@ Side Windows
 @node Displaying Buffers in Side Windows
 @subsection Displaying Buffers in Side Windows

-The following action function for @code{display-buffer} (@pxref{Display
-Action Functions}) creates or reuses a side window for displaying the
+The following action function for @code{display-buffer} (@pxref{Action
+Functions}) creates or reuses a side window for displaying the
 specified buffer.

 @defun display-buffer-in-side-window buffer alist
@@ -3264,7 +3964,7 @@ Displaying Buffers in Side Windows

 By default, side windows cannot be split via @code{split-window}
 (@pxref{Splitting Windows}).  Also, a side window is not reused or split
-by any buffer display action (@pxref{Display Action Functions}) unless
+by any buffer display action (@pxref{Action Functions}) unless
 it is explicitly specified as target of that action.  Note also that
 @code{delete-other-windows} cannot make a side window the only window on
 its frame (@pxref{Deleting Windows}).
@@ -3453,9 +4153,9 @@ Frame Layouts with Side Windows
 @xref{Resizing Windows}.

    The last form also makes sure that none of the created side windows
-are accessible via @kbd{C-x o} by installing the @code{no-other-window}
+are accessible via @w{@kbd{C-x o}} by installing the @code{no-other-window}
 parameter for each of these windows.  In addition, it makes sure that
-side windows are not deleted via @kbd{C-x 1} by installing the
+side windows are not deleted via @w{@kbd{C-x 1}} by installing the
 @code{no-delete-other-windows} parameter for each of these windows.

    Since @code{dired} buffers have no fixed names, we use a special
@@ -3547,7 +4247,7 @@ Atomic Windows

 To create a new atomic window from an existing live window or to add a
 new window to an existing atomic window, the following buffer display
-action function (@pxref{Display Action Functions}) can be used:
+action function (@pxref{Action Functions}) can be used:

 @defun display-buffer-in-atom-window buffer alist
 This function tries to display @var{buffer} in a new window that will be
@@ -3679,6 +4379,7 @@ Window Point
 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
@@ -3925,6 +4626,7 @@ Window Start and End
 text line, @var{ypos} is negative.
 @end defun

+
 @node Textual Scrolling
 @section Textual Scrolling
 @cindex textual scrolling
@@ -4264,6 +4966,7 @@ Vertical Scrolling
 presence of large images.
 @end defvar

+
 @node Horizontal Scrolling
 @section Horizontal Scrolling
 @cindex horizontal scrolling
@@ -5145,6 +5848,7 @@ Window Parameters
 versions of Emacs.
 @end table

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

^ permalink raw reply related	[flat|nested] 32+ messages in thread
end of thread, other threads:[~2018-11-08 19:25 UTC | newest]

Thread overview: 32+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2018-10-20 12:20 Documenting buffer display martin rudalics
2018-10-20 13:21 ` Eli Zaretskii
2018-10-20 18:02   ` martin rudalics
2018-10-21 12:56     ` Eli Zaretskii
2018-10-22  9:06       ` martin rudalics
2018-10-22 13:55         ` Eli Zaretskii
2018-10-22 19:14           ` martin rudalics
2018-10-22 19:27             ` Eli Zaretskii
2018-10-23  8:58               ` martin rudalics
2018-10-23 11:26                 ` Pierre-Yves Luyten
2018-10-23 13:45                   ` martin rudalics
2018-10-23 17:40                   ` Stefan Monnier
2018-10-23 14:04                 ` Drew Adams
2018-10-23 18:18                   ` martin rudalics
2018-10-23 15:18                 ` Eli Zaretskii
2018-10-23 18:23                   ` martin rudalics
2018-10-23 19:07                     ` Eli Zaretskii
2018-10-24  9:44                       ` martin rudalics
2018-10-24 14:48                         ` Eli Zaretskii
2018-10-24 17:40                           ` martin rudalics
2018-10-24 18:25                             ` Eli Zaretskii
2018-10-25 20:42                             ` Juri Linkov
2018-10-23 15:52                 ` Alan Mackenzie
2018-10-23 18:25                   ` martin rudalics
2018-11-08 19:25                   ` martin rudalics
2018-10-22  1:39   ` Michael Welsh Duggan
2018-10-22  5:54     ` Eli Zaretskii
2018-10-20 15:22 ` Drew Adams
2018-10-20 18:02   ` martin rudalics
2018-10-20 18:24     ` Drew Adams
2018-10-21  8:22       ` martin rudalics
2018-11-04  9:06 ` martin rudalics

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