bug#51883: 29.0.50; Command to get accidentally deleted frames back

[Gregory Heytings <gregory@heytings.org>]

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Updated patch attached.

(A note for Michael: I know about the ring.el library, but it would have 
been necessary to require it in frame.el, which is not necessary, given 
that it's easy to get the same effect with standard functions.)

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From 69657897097c986efcc859a070fbd850989c8343 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Sat, 27 Nov 2021 16:59:51 +0000
Subject: [PATCH] Undelete deleted frames.

* lisp/frame.el (undelete-frame): New command.
(undelete-frame--handle-delete-frame): New auxiliary function.
(undelete-frame--deleted-frames): New auxiliary variables.
(undelete-frame-mode): New minor mode.
(ctl-x-5-map): Bind the new command.

* etc/NEWS: Document the new command and minor mode.

* src/frame.c (Fdelete_frame): Update docstring, and mention the
minor mode.

* lisp/menu-bar.el (menu-bar-file-menu): Add an entry for the
new command.

* doc/emacs/frames.tex (Frame Commands): Document the new command
and minor mode.

See bug#51883.
---
 doc/emacs/frames.texi | 10 ++++++
 etc/NEWS              | 10 ++++++
 lisp/frame.el         | 75 +++++++++++++++++++++++++++++++++++++++++++
 lisp/menu-bar.el      |  9 ++++++
 src/frame.c           |  5 ++-
 5 files changed, 108 insertions(+), 1 deletion(-)

diff --git a/doc/emacs/frames.texi b/doc/emacs/frames.texi
index c14ada2957..b0fa22660f 100644
--- a/doc/emacs/frames.texi
+++ b/doc/emacs/frames.texi
@@ -512,6 +512,16 @@ Frame Commands
 Delete the selected frame (@code{delete-frame}).  This signals an
 error if there is only one frame.
 
+@item C-x 5 u
+@kindex C-x 5 u
+@findex undelete-frame
+@findex undelete-frame-mode
+When @code{undelete-frame-mode} is enabled, undelete one of the 16
+most recently deleted frames.  Without a prefix argument, undelete the
+most recently deleted frame.  With a numerical prefix argument between
+1 and 16, where 1 is the most recently deleted frame, undelete the
+corresponding deleted frame.
+
 @item C-z
 @kindex C-z @r{(X windows)}
 Minimize (or iconify) the selected Emacs frame
diff --git a/etc/NEWS b/etc/NEWS
index 8b7c2f7850..45628b6a65 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -189,6 +189,16 @@ For example, a 'display-buffer-alist' entry of
 will make the body of the chosen window 40 columns wide.  For the
 height use 'window-height' in combination with 'body-lines'.
 
+** Frames
+
++++
+*** Deleted frames can now be undeleted.
+The 16 most recently deleted frames can be undeleted with C-x 5 u when
+undelete-frame-mode is enabled.  Without a prefix argument, undelete
+the most recently deleted frame.  With a numerical prefix argument
+between 1 and 16, where 1 is the most recently deleted frame, undelete
+the corresponding deleted frame.
+
 ** Tab Bars and Tab Lines
 
 ---
diff --git a/lisp/frame.el b/lisp/frame.el
index 1319759e74..22e373fa0e 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2499,6 +2499,80 @@ delete-other-frames
         (if iconify (iconify-frame this) (delete-frame this)))
       (setq this next))))
 
+(eval-when-compile (require 'frameset))
+
+(defvar undelete-frame--deleted-frames nil
+  "Internal variable used by `undelete-frame--handle-delete-frame'.")
+
+(defun undelete-frame--handle-delete-frame (frame)
+  "Save the configuration of frames deleted with `delete-frame'.
+Only the 16 most recently deleted frames are saved."
+  (when (frame-live-p frame)
+    (setq undelete-frame--deleted-frames
+          (cons
+           (cons
+            (display-graphic-p)
+            (frameset-save
+             (list frame)
+             ;; When the daemon is started from a graphical
+             ;; environment, TTY frames have a 'display' parameter set
+             ;; to the value of $DISPLAY (see the note in
+             ;; `server--on-display-p').  Do not store that parameter
+             ;; in the frameset, otherwise `frameset-restore' attempts
+             ;; to restore a graphical frame.
+             :filters (if (display-graphic-p)
+                          frameset-filter-alist
+                        (cons '(display . :never)
+                              frameset-filter-alist))))
+           undelete-frame--deleted-frames))
+    (if (> (length undelete-frame--deleted-frames) 16)
+        (setq undelete-frame--deleted-frames
+              (butlast undelete-frame--deleted-frames)))))
+
+(define-minor-mode undelete-frame-mode
+  "Enable the `undelete-frame' command."
+  :group 'frames
+  :global t
+  (if undelete-frame-mode
+      (add-hook 'delete-frame-functions
+                #'undelete-frame--handle-delete-frame -75)
+    (remove-hook 'delete-frame-functions
+                 #'undelete-frame--handle-delete-frame)
+    (setq undelete-frame--deleted-frames nil)))
+
+(defun undelete-frame (&optional arg)
+  "Undelete a frame deleted with `delete-frame'.
+Without a prefix argument, undelete the most recently deleted
+frame.
+With a numerical prefix argument ARG between 1 and 16, where 1 is
+most recently deleted frame, undelete the ARGth deleted frame.
+When called from Lisp, returns the new frame."
+  (interactive "P")
+  (if (not undelete-frame-mode)
+      (error "Undelete-Frame mode is disabled")
+    (if (consp arg)
+        (error "Missing deleted frame number argument")
+      (let* ((number (or arg 1))
+             (frames (frame-list))
+             (frameset (nth (1- number) undelete-frame--deleted-frames))
+             (graphic (display-graphic-p)))
+        (if (not (<= 1 number 16))
+            (error "%d is not a valid deleted frame number argument"
+                   number)
+          (if (not frameset)
+              (error "No deleted frame with number %d" number)
+            (if (not (eq graphic (car frameset)))
+                (error
+                 "Cannot undelete a %s display frame on a %s display"
+                 (if graphic "non-graphic" "graphic")
+                 (if graphic "graphic" "non-graphic"))
+              (setq undelete-frame--deleted-frames
+                    (delq frameset undelete-frame--deleted-frames))
+              (frameset-restore (cdr frameset))
+              (let ((frame (car (seq-difference (frame-list) frames))))
+                (when frame
+                  (select-frame-set-input-focus frame)
+                  frame)))))))))
 \f
 ;;; Window dividers.
 (defgroup window-divider nil
@@ -2843,6 +2917,7 @@ ctl-x-5-map
 (define-key ctl-x-5-map "o" #'other-frame)
 (define-key ctl-x-5-map "5" #'other-frame-prefix)
 (define-key ctl-x-5-map "c" #'clone-frame)
+(define-key ctl-x-5-map "u" #'undelete-frame)
 (define-key global-map [f11] #'toggle-frame-fullscreen)
 (define-key global-map [(meta f10)] #'toggle-frame-maximized)
 (define-key esc-map    [f10]        #'toggle-frame-maximized)
diff --git a/lisp/menu-bar.el b/lisp/menu-bar.el
index 274f594f69..9500f577bc 100644
--- a/lisp/menu-bar.el
+++ b/lisp/menu-bar.el
@@ -109,6 +109,15 @@ menu-bar-file-menu
       (bindings--define-key menu [separator-tab]
         menu-bar-separator))
 
+    (bindings--define-key menu [enable-undelete-frame-mode]
+      '(menu-item "Enable Frame Undeletion" undelete-frame-mode
+                  :visible (null undelete-frame-mode)
+                  :help "Enable frame undeletion for this session"))
+    (bindings--define-key menu [undelete-last-deleted-frame]
+      '(menu-item "Undelete Frame" undelete-frame
+                  :visible undelete-frame-mode
+                  :help "Undelete the most recently deleted frame"))
+
     ;; Don't use delete-frame as event name because that is a special
     ;; event.
     (bindings--define-key menu [delete-this-frame]
diff --git a/src/frame.c b/src/frame.c
index 33e9606e41..04dd6e3550 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -2376,9 +2376,12 @@ delete_frame (Lisp_Object frame, Lisp_Object force)
 }
 
 DEFUN ("delete-frame", Fdelete_frame, Sdelete_frame, 0, 2, "",
-       doc: /* Delete FRAME, permanently eliminating it from use.
+       doc: /* Delete FRAME, eliminating it from use.
 FRAME must be a live frame and defaults to the selected one.
 
+When `undelete-frame-mode' is enabled, the 16 most recently deleted
+frames can be undeleted with `undelete-frame', which see.
+
 A frame may not be deleted if its minibuffer serves as surrogate
 minibuffer for another frame.  Normally, you may not delete a frame if
 all other frames are invisible, but if the second optional argument
-- 
2.33.0