Document and fix some bugs with side windows

Add a documentation for side windows and fix some bugs
found when testing their behavior.  Also add a new window
parameter `no-delete-other-window', a new `display-buffer'
alist member called `window-parameters', and functions to
toggle and reverse side windows on a frame.  Add new function
`window-swap-states' to exchange states of two live windows.

* lisp/window.el (display-buffer-in-atom-window): Use
`split-window-no-error'.
(window-sides-vertical): Maybe change layouts when setting this
variable.
(window-sides-reversed): New option.
(window-sides-slots): Rewrite doc-string and help echoes.
(window-sides-shown): New buffer-local variable set when showing
a buffer in a side window.
(window--sides-inhibit-check): New variable.
(window--sides-reverse-on-frame-p, window-toggle-side-windows)
(window--sides-reverse-all, window--sides-reverse-frame)
(window--sides-reverse-side, window--sides-reverse)
(window--sides-verticalize-frame, window--sides-verticalize)
(window--sides-check-failed): New functions.
(window--side-window-p): Remove function.
(window--major-non-side-window): Rename to `window-main-window',
adjust callers, rewrite doc-string.
(window--major-side-window): Rename to
`window--make-major-side-window-next-to', adjust caller, fix
doc-string.
(display-buffer-in-major-side-window): Rename to
`window--make-major-side-window', adjust
caller, rewrite doc-string.  Make `window-side' and
`window-slot' parameters persistent (Bug#23858).  Don't set
`delete-window' parameter.  Add `preserve-size' entry to ALIST.
(delete-side-window): Remove function.
(display-buffer-in-side-window): Fix doc-string.  Don't set
`delete-window' parameter.  Add `preserve-size' entry to ALIST.
(window--side-check): Rename to window--sides-check.  Rewrite
completely.  Adjust caller.
(window-resize-no-error): Don't describe PIXELWISE argument.
(adjust-window-trailing-edge): Fix bug that disallowed
re-enlarging windows that were too small.
(window-deletable-p): Don't tell that a minibuffer window on a
non-minibuffer-only frame can be deleted.  Fix doc-string.
(delete-window): Handle deleting a side window here (the
`delete-window' parameter is no more set for side windows).
(delete-other-windows): Handle ‘no-delete-other-window'
parameter. Don't treat side windows separately (see discussion
of Bug#24368) but keep optimization that makes the main window
the root window of its frame.
(switch-to-prev-buffer, switch-to-next-buffer): Handle side
windows and buffers shown in side windows separately.
(split-window-no-error): New function.
(window--state-get-1): Use right buffer when storing window
point and start positions and WRITABLE is nil (Bug#24368).
(window--state-put-1): Fix handling of `window-combination-limit'.
Use `split-window-no-error'.
(window--state-put-2): Try to restore windows with preserved
size to their original size.  Fix bug where a fixed window's width
was not preserved.
(window-state-put): When reducing an internal window to a live
one, don't choose a side window.
(window-swap-states): New function.
(window-splittable-p): Don't call `window--side-window-p'.
(window--display-buffer): Handle `window-parameters' ALIST entry.
Minor rewrite.
(display-buffer): Mention `window-parameters' entry in
doc-string.
(display-buffer-at-bottom): Call `split-window-no-error'.

* doc/lispref/elisp.texi (Top): New section "Side Windows".
* doc/lispref/windows.texi (Deleting Windows): Fix descriptions
of `delete-window' and `delete-other-windows' wrt window
parameters and side windows.
(Display Action Functions): Mention `window-parameters' ALIST
entry.
(Side Windows): New section (Bug#18170).
(Window Configurations): Describe new function `window-swap-states'.
(Window Parameters): Say that functions may behave specially when
their homonymous window parameter has been set.  Mention new
parameter `no-delete-other-window'.  Add cross reference for
`window-side' and `window-slot' parameters.

doc/lispref/elisp.texi

@@ -1038,6 +1038,7 @@ Windows
                               a specific window.
 * Quitting Windows::        How to restore the state prior to displaying a
                               buffer.
+* Side Windows::            Special windows on a frame's sides.
 * Window Point::            Each window has its own location of point.
 * Window Start and End::    Buffer positions indicating which text is
                               on-screen in a window.
@@ -1051,6 +1052,14 @@ Windows
                               redisplay going past a certain point,
                               or window configuration changes.
 
+Side Windows
+
+* Displaying Buffers in Side Windows:: An action function for displaying
+                              buffers in side windows.
+* Side Window Options and Functions:: Further tuning of side windows.
+* Frame Layouts with Side Windows:: Setting up frame layouts with side
+                              windows.
+
 Frames
 
 * Creating Frames::         Creating additional frames.

doc/lispref/windows.texi

@@ -33,6 +33,7 @@ is displayed in windows.
                               a specific window.
 * Quitting Windows::        How to restore the state prior to displaying a
                               buffer.
+* Side Windows::            Special windows on a frame's sides.
 * Window Point::            Each window has its own location of point.
 * Window Start and End::    Buffer positions indicating which text is
                               on-screen in a window.
@@ -1275,9 +1276,12 @@ Configurations}).
 @deffn Command delete-window &optional window
 This function removes @var{window} from display and returns
 @code{nil}.  If @var{window} is omitted or @code{nil}, it defaults to
-the selected window.  If deleting the window would leave no more
-windows in the window tree (e.g., if it is the only live window in the
-frame), an error is signaled.
+the selected window.
+
+If deleting the window would leave no more windows in the window tree
+(e.g., if it is the only live window in the frame) or all remaining
+windows on @var{window}'s frame are side windows (@pxref{Side Windows}),
+an error is signaled.
 
 By default, the space taken up by @var{window} is given to one of its
 adjacent sibling windows, if any.  However, if the variable
@@ -1285,33 +1289,34 @@ adjacent sibling windows, if any.  However, if the variable
 proportionally distributed among any remaining windows in the same
 window combination.  @xref{Recombining Windows}.
 
-The behavior of this function may be altered by the window parameters
-of @var{window}, so long as the variable
-@code{ignore-window-parameters} is @code{nil}.  If the value of
-the @code{delete-window} window parameter is @code{t}, this function
-ignores all other window parameters.  Otherwise, if the value of the
-@code{delete-window} window parameter is a function, that function is
-called with the argument @var{window}, in lieu of the usual action of
-@code{delete-window}.  Otherwise, this function obeys the
-@code{window-atom} or @code{window-side} window parameter, if any.
-@xref{Window Parameters}.
+The behavior of this function may be altered by the window parameters of
+@var{window}, so long as the variable @code{ignore-window-parameters} is
+@code{nil}.  If the value of the @code{delete-window} window parameter
+is @code{t}, this function ignores all other window parameters.
+Otherwise, if the value of the @code{delete-window} window parameter is
+a function, that function is called with the argument @var{window}, in
+lieu of the usual action of @code{delete-window}.  @xref{Window
+Parameters}.
 @end deffn
 
 @deffn Command delete-other-windows &optional window
-This function makes @var{window} fill its frame, by deleting other
-windows as necessary.  If @var{window} is omitted or @code{nil}, it
-defaults to the selected window.  The return value is @code{nil}.
+This function makes @var{window} fill its frame, deleting other windows
+as necessary.  If @var{window} is omitted or @code{nil}, it defaults to
+the selected window.  An error is signaled if @var{window} is a side
+window (@pxref{Side Windows}).  The return value is @code{nil}.
 
-The behavior of this function may be altered by the window parameters
-of @var{window}, so long as the variable
-@code{ignore-window-parameters} is @code{nil}.  If the value of
-the @code{delete-other-windows} window parameter is @code{t}, this
-function ignores all other window parameters.  Otherwise, if the value
-of the @code{delete-other-windows} window parameter is a function,
-that function is called with the argument @var{window}, in lieu of the
-usual action of @code{delete-other-windows}.  Otherwise, this function
-obeys the @code{window-atom} or @code{window-side} window parameter,
-if any.  @xref{Window Parameters}.
+The behavior of this function may be altered by the window parameters of
+@var{window}, so long as the variable @code{ignore-window-parameters} is
+@code{nil}.  If the value of the @code{delete-other-windows} window
+parameter is @code{t}, this function ignores all other window
+parameters.  Otherwise, if the value of the @code{delete-other-windows}
+window parameter is a function, that function is called with the
+argument @var{window}, in lieu of the usual action of
+@code{delete-other-windows}.  @xref{Window Parameters}.
+
+Also, if @code{ignore-window-parameters} is @code{nil}, this function
+does not delete any window whose @code{no-delete-other-window} parameter
+is non-@code{nil}.
 @end deffn
 
 @deffn Command delete-windows-on &optional buffer-or-name frame
@@ -2574,7 +2579,12 @@ assumed that when the caller specifies a non-@code{nil}
 from @code{display-buffer} in this case.
 @end defun
 
-To illustrate the use of action functions, consider the following
+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
@@ -3068,6 +3078,295 @@ other frame on the same terminal.
 @end defopt
 
 
+@node Side Windows
+@section Side Windows
+@cindex side windows
+@cindex main window
+@cindex main window of a frame
+
+Side windows are special windows positioned at any of the four sides of
+a frame's root window (@pxref{Windows and Frames}).  In practice, this
+means that the area of the frame's root window is subdivided into a main
+window and a number of side windows surrounding that main window.  The
+main window is either a ``normal'' live window or specifies the area
+containing all the normal 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
+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).
+
+@menu
+* Displaying Buffers in Side Windows:: An action function for displaying
+                              buffers in side windows.
+* Side Window Options and Functions:: Further tuning of side windows.
+* Frame Layouts with Side Windows:: Setting up frame layouts with side
+                              windows.
+@end menu
+
+
+@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
+specified buffer.
+
+@defun display-buffer-in-side-window buffer alist
+This function displays @var{buffer} in a side window of the selected
+frame.  @var{alist} is an association list of symbols and values as for
+@code{display-buffer}.  The following symbols in @var{alist} are special
+for this function:
+
+@table @code
+@item side
+Denotes the side of the frame where the window shall be located.  Valid
+values are @code{left}, @code{top}, @code{right} and @code{bottom}.  If
+unspecified, the window is located at the bottom of the frame.
+
+@item slot
+Denotes a slot at the specified side where to locate the window.  A
+value of zero means to preferably position the window in the middle of
+the specified side.  A negative value means to use a slot preceding
+(that is, above or on the left of) the middle slot.  A positive value
+means to use a slot following (that is, below or on the right of) the
+middle slot.  Hence, all windows on a specific side are ordered by their
+@code{slot} value.  If unspecified, the window is located in the middle
+of the specified side.
+@end table
+
+If you specify the same slot on the same side for two or more different
+buffers, the buffer displayed last is shown in the corresponding window.
+Hence, slots can be used for sharing the same side window between
+buffers.
+
+This function installs the @code{window-side} and @code{window-slot}
+parameters (@pxref{Window Parameters}) and makes them persistent.  It
+does not install any other window parameters unless they have been
+explicitly provided via a @code{window-parameters} entry in @var{alist}.
+@end defun
+
+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
+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}).
+
+   Once set up, side windows also change the behavior of the commands
+@code{switch-to-prev-buffer} and @code{switch-to-next-buffer}
+(@pxref{Window History}).  In particular, these commands will refrain
+from showing, in a side window, buffers that have not been displayed in
+that window before.  And, they will refrain from having a normal,
+non-side window show a buffer that has been already displayed in a side
+window.  A notable exception to the latter rule occurs when an
+application, after displaying a buffer, resets that buffer's local
+variables.
+
+
+@node Side Window Options and Functions
+@subsection Side Window Options and Functions
+
+The following options provide additional control over the placement of
+side windows.
+
+@defopt window-sides-vertical
+If non-@code{nil}, this means that the side windows on the left and
+right of a frame occupy the frame's full height.  Otherwise, the side
+windows on the top and bottom of the frame occupy the frame's full
+width.
+@end defopt
+
+@defopt window-sides-slots
+This option specifies the maximum number of side windows on each side of
+a frame.  The value is a list of four elements specifying the number of
+side window slots on (in this order) the left, top, right and bottom of
+each frame.  If an element is a number, this means to display at most
+that many windows on the corresponding side.  If an element is
+@code{nil}, this means there's no bound on the number of slots on that
+side.
+
+If any of the specified values is zero, no window can be created on the
+corresponding side.  @code{display-buffer-in-side-window} will not
+signal an error in that case but return @code{nil}.  If a specified
+value just forbids the creation of an additional side window, the most
+suitable window on that side is reused and may have its
+@code{window-slot} parameter changed accordingly.
+@end defopt
+
+@defopt window-sides-reversed
+This option specifies whether top/bottom side windows should appear in
+reverse order.  When this is @code{nil}, side windows on the top and
+bottom of a frame are always drawn from left to right with increasing
+slot values.  When this is @code{t}, the drawing order is reversed and
+side windows on the top and bottom of a frame are drawn from right to
+left with increasing slot values.
+
+When this is @code{bidi}, the drawing order is reversed if and only if
+the value of @code{bidi-paragraph-direction} is @code{right-to-left} in
+the buffer displayed in the window most recently selected within the
+main window area of this frame.  Sometimes that window may be hard to
+find, so heuristics are used to avoid that the drawing order changes
+inadvertently when another window gets selected.
+
+The layout of side windows on the left or right of a frame is not
+affected by the value of this variable.
+@end defopt
+
+When a frame has side windows, the following function returns the main
+window of that frame.
+
+@defun window-main-window &optional frame
+This function returns the main window of the specified @var{frame}.  The
+optional argument @var{frame} must be a live frame and defaults to the
+selected one.
+
+If @var{frame} has no side windows, it returns @var{frame}'s root
+window.  Otherwise, it returns either an internal non-side window such
+that all other non-side windows on @var{frame} descend from it, or the
+single live non-side window of @var{frame}.  Note that the main window
+of a frame cannot be deleted via @code{delete-window}.
+@end defun
+
+The following command is handy to toggle the appearance of all side
+windows on a specified frame.
+
+@deffn Command window-toggle-side-windows &optional frame
+This command toggles side windows on the specified @var{frame}.  The
+optional argument @var{frame} must be a live frame and defaults to the
+selected one.
+
+If @var{frame} has at least one side window, this command saves the
+state of @var{frame}'s root window in the @var{frame}'s
+@code{window-state} frame parameter and deletes all side windows on
+@var{frame} afterwards.
+
+If @var{frame} has no side window but a @code{window-state} parameter,
+it uses that parameter's value to restore the side windows on
+@var{frame} leaving @var{frame}'s main window alone.
+
+An error is signaled if @var{frame} has no side window and no saved
+state is found.
+@end deffn
+
+
+@node Frame Layouts with Side Windows
+@subsection Frame Layouts with Side Windows
+
+Side windows can be used to create more complex frame layouts like those
+provided by integrated development environments (IDEs).  In such
+layouts, the area of the main window is where the normal editing
+activities take place.  Side windows are not conceived for editing in
+the usual sense.  Rather, they are supposed to display information
+complementary to the current editing activity, like lists of files, tags
+or buffers, help information, search or grep results or shell output.
+
+   The layout of such a frame might appear as follows:
+
+@smallexample
+@group
+     ___________________________________
+    |          *Buffer List*            |
+    |___________________________________|
+    |     |                       |     |
+    |  *  |                       |  *  |
+    |  d  |                       |  T  |
+    |  i  |                       |  a  |
+    |  r  |   Main Window Area    |  g  |
+    |  e  |                       |  s  |
+    |  d  |                       |  *  |
+    |  *  |                       |     |
+    |_____|_______________________|_____|
+    | *help*/*grep*/  |  *shell*/       |
+    | *Completions*   |  *compilation*  |
+    |_________________|_________________|
+    |             Echo Area             |
+    |___________________________________|
+
+
+@end group
+@end smallexample
+
+The following example illustrates how window parameters (@pxref{Window
+Parameters}) can be used with @code{display-buffer-in-side-window}
+(@pxref{Displaying Buffers in Side Windows}) to set up code for
+producing the frame sketched above.
+
+@example
+@group
+(defvar parameters
+  '(window-parameters . ((no-other-window . t) (no-delete-other-window . t))))
+
+(setq fit-window-to-buffer-horizontally t)
+(setq window-resize-pixelwise t)
+
+(setq
+ display-buffer-alist
+ `(("\\*Buffer List\\*" display-buffer-in-side-window
+    (side . top) (slot . 0) (window-height . fit-window-to-buffer)
+    (preserve-size . (nil . t)) ,parameters)
+   ("\\*Tags List\\*" display-buffer-in-side-window
+    (side . right) (slot . 0) (window-width . fit-window-to-buffer)
+    (preserve-size . (t . nil)) ,parameters)
+   ("\\*\\(?:help\\|grep\\|Completions\\)\\*" display-buffer-in-side-window
+    (side . bottom) (slot . -1) (preserve-size . (nil . t)) ,parameters)
+   ("\\*\\(?:shell\\|compilation\\)\\*" display-buffer-in-side-window
+    (side . bottom) (slot . 1) (preserve-size . (nil . t)) ,parameters)))
+@end group
+@end example
+
+This specifies @code{display-buffer-alist} entries (@pxref{Choosing
+Window}) for buffers with fixed names.  In particular, it asks for
+showing @file{*Buffer List*} with adjustable height at the top of the
+frame and @file{*Tags List*} with adjustable width on the frame's right.
+It also asks for having the @file{*help*}, @file{*grep*} and
+@file{*Completions*} buffers share a window on the bottom left side of
+the frame and the @file{*shell*} and @file{*compilation*} buffers appear
+in a window on the bottom right side of the frame.
+
+   Note that the option @code{fit-window-to-buffer-horizontally} must
+have a non-@code{nil} value in order to allow horizontal adjustment of
+windows.  We also added entries that ask for preserving the height of
+side windows at the top and bottom of the frame and the width of side
+windows at the left or right of the frame.  To assure that side windows
+retain their respective sizes when maximizing the associated frame, we
+have also set the variable @code{window-resize-pixelwise}.
+@xref{Resizing Windows}.
+
+   The last form also makes sure that none of the side windows it
+creates are accessible via @kbd{C-x o} by installing a
+@code{no-other-window} parameter for each of these windows.  In addition
+it also makes sure that side windows are not deleted via @kbd{C-x 1} by
+installing a @code{no-delete-other-window} parameter on each of these
+windows.
+
+   Since @code{dired} buffers have no fixed names, we use a special
+function @code{dired-default-directory-on-left} in order to display a
+lean directory buffer on the left side of the frame.
+
+@example
+@group
+(defun dired-default-directory-on-left ()
+  "Display `default-directory' in side window on left, hiding details."
+  (interactive)
+  (let ((buffer (dired-noselect default-directory)))
+    (with-current-buffer buffer (dired-hide-details-mode t))
+    (display-buffer-in-side-window
+     buffer `((side . left) (slot . 0)
+              (window-width . fit-window-to-buffer)
+              (preserve-size . (t . nil)) ,parameters))))
+@end group
+@end example
+
+Evaluating the preceding forms and typing, in any order, @kbd{M-x
+list-buffers}, @kbd{C-h f}, @kbd{M-x shell}, @kbd{M-x list-tags} and
+@kbd{M-x dired-default-directory-on-left} should now reproduce the frame
+layout sketched above.
+
+
 @node Window Point
 @section Windows and Point
 @cindex window position
@@ -4242,6 +4541,25 @@ is @code{safe}, this means windows can get as small as one line
 and/or two columns.
 @end defun
 
+The functions @code{window-state-get} and @code{window-state-put} also
+allow to exchange the contents of two live windows.  The following
+function does precisely that:
+
+@deffn Command window-swap-states &optional window-1 window-2 size
+This command swaps the states of the two live windows @var{window-1} and
+@var{window-2}.  @var{window-1} must specify a live window and defaults
+to the selected one.  @var{window-2} must specify a live window and
+defaults to the window following @var{window-1} in the cyclic ordering
+of windows, excluding minibuffer windows and including live windows on
+all visible frames.
+
+Optional argument @var{size} non-@code{nil} means to try swapping the
+sizes of @var{window-1} and @var{window-2} as well.  A value of
+@code{height} means to swap heights only, a value of @code{width}
+means to swap widths only, while @code{t} means to swap both widths
+and heights, if possible.  Frames are not resized by this function.
+@end deffn
+
 
 @node Window Parameters
 @section Window Parameters
@@ -4279,6 +4597,7 @@ earlier by @code{window-state-get}, all cloned windows have their
 parameters reset to @code{nil}.  The following variable allows you to
 override the standard behavior:
 
+@cindex persistent window parameters
 @defvar window-persistent-parameters
 This variable is an alist specifying which parameters get saved by
 @code{current-window-configuration} and @code{window-state-get}, and
@@ -4308,10 +4627,10 @@ may fail with an @code{invalid-read-syntax} error.
 @end defvar
 
 Some functions (notably @code{delete-window},
-@code{delete-other-windows} and @code{split-window}), may behave specially
-when their @var{window} argument has a parameter set.  You can override
-such special behavior by binding the following variable to a
-non-@code{nil} value:
+@code{delete-other-windows} and @code{split-window}), may behave
+specially when the window specified by their @var{window} argument has
+the homonymous parameter set.  You can override such special behavior by
+binding the following variable to a non-@code{nil} value:
 
 @defvar ignore-window-parameters
 If this variable is non-@code{nil}, some standard functions do not
@@ -4337,6 +4656,10 @@ This parameter affects the execution of @code{delete-window}
 This parameter affects the execution of @code{delete-other-windows}
 (@pxref{Deleting Windows}).
 
+@item @code{no-delete-other-window}
+This parameter marks the window as not deletable by
+@code{delete-other-windows} (@pxref{Deleting Windows}).
+
 @item @code{split-window}
 This parameter affects the execution of @code{split-window}
 (@pxref{Splitting Windows}).
@@ -4387,6 +4710,10 @@ The fourth element is the buffer whose display caused the creation of
 this parameter.  @code{quit-restore-window} deletes the specified window
 only if it still shows that buffer.
 
+@item @code{window-side} @code{window-slot}
+These parameters are used for implementing side windows (@pxref{Side
+Windows}).
+
 @item @code{min-margins}
 The value of this parameter is a cons cell whose @sc{car} and @sc{cdr},
 if non-@code{nil}, specify the minimum values (in columns) for the left
@@ -4409,8 +4736,7 @@ applications.  It might be replaced by an improved solution in future
 versions of Emacs.
 @end table
 
-There are additional parameters @code{window-atom} and @code{window-side};
-these are reserved and should not be used by applications.
+The @code{window-atom} parameter is used for implemeting atomic windows.
 
 
 @node Window Hooks

etc/NEWS

@@ -611,6 +611,27 @@ collection).
 ** The new functions 'make-nearby-temp-file' and 'temporary-file-directory'
 can be used for creation of temporary files of remote or mounted directories.
 
+** Changes in Frame- and Window- Handling
+
++++
+*** Support for side windows is now official.  The display action
+function `display-buffer-in-side-window' will display its buffer in a
+side window.  Functions for toggling all side windows on a frame,
+changing and reversing the layout of side windows and returning the main
+(major non-side) window of a frame are provided.  For details consult
+the section "Side Windows" in the Elisp manual.
+
++++
+*** New `display-buffer' alist entry `window-parameters' allows to
+assign window parameters to the window used for displaying the buffer.
+
++++
+*** New window parameter `no-delete-other-window' prevents that
+its window gets deleted by `delete-other-windows'.
+
++++
+*** New command `window-swap-states' swaps the states of two live
+windows.
 
 * Changes in Emacs 26.1 on Non-Free Operating Systems