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

[Juri Linkov <juri@linkov.net>]

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> Eli Zaretskii [2021-11-16 19:40:08] wrote:
>>> +(add-hook 'delete-frame-functions #'undeleted-frame--save-deleted-frame)
>>
>> I'd rather we didn't do that by default.  Several reasons:
>>
>>   . the startup code deletes the terminal frame, so the above means we
>>     will always load frameset, which is not a small package, at
>>     startup, even if the user has no use for this functionality
>
> Clearly we should avoid putting the special initial-frame in the undo
> log since we don't want (and can't) recreate it later anyway.

This is fixed now.

> But as for loading the package, the problem is more general in the sense
> that maybe we should refrain from loading the package just because
> a frame is deleted.
>
> So maybe we should change the code so the hook's function doesn't
> require loading `frameset.el`.  AFAICT fundamentally, the only function
> from `frameset.el` that it needs is `frameset-filter-params`, so maybe
> we should move some of that code to `frame.el`.

Now the undelete-frame feature doesn't load frameset.el anymore.

So all concerns raised by Eli are addressed now
that make possible to enable it by default
as was discussed on emacs-devel:


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diff --git a/doc/emacs/frames.texi b/doc/emacs/frames.texi
index 29edbe9863..39b38a95d3 100644
--- a/doc/emacs/frames.texi
+++ b/doc/emacs/frames.texi
@@ -515,12 +515,14 @@ Frame Commands
 @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.
+@findex undelete-frame-max
+Undelete one of the recently deleted frames.  The user option
+@code{undelete-frame-max} specifies the maximum number of deleted
+frames to keep (the default is 1).  Without a prefix argument,
+undelete the most recently deleted frame.  With a numerical prefix
+argument between 1 and the number specified by @code{undelete-frame-max},
+where 1 is the most recently deleted frame, undelete the corresponding
+deleted frame.
 
 @item C-z
 @kindex C-z @r{(X windows)}
diff --git a/etc/NEWS b/etc/NEWS
index 19d23e8943..385ddb0262 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -327,11 +327,13 @@ height use 'window-height' in combination with 'body-lines'.
 
 +++
 *** 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.
+The most recently deleted frame can be undeleted with 'C-x 5 u'
+if the new user option 'undelete-frame-max' is a positive number.
+This option specifies the maximum number of closed frames to remember.
+The default is 1.  Without a prefix argument, undelete the most
+recently deleted frame.  With a numerical prefix argument between 1
+and the value of 'undelete-frame-max', 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 d39597d0af..143c51b98c 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2530,12 +2530,19 @@ delete-other-frames
         (if iconify (iconify-frame this) (delete-frame this)))
       (setq this next))))
 
+\f
+(defcustom undelete-frame-max 1
+  "Maximum number of deleted frames before oldest are thrown away."
+  :type 'integer
+  :group 'frames
+  :version "29.1")
+
 (defvar undelete-frame--deleted-frames nil
   "Internal variable used by `undelete-frame--save-deleted-frame'.")
 
 (defun undelete-frame--save-deleted-frame (frame)
   "Save the configuration of frames deleted with `delete-frame'.
-Only the 16 most recently deleted frames are saved."
+Only the `undelete-frame-max' most recently deleted frames are saved."
   (when (and after-init-time (frame-live-p frame))
     (setq undelete-frame--deleted-frames
           (cons
@@ -2554,53 +2561,41 @@ undelete-frame--save-deleted-frame
              (frame-parameters frame))
             (window-state-get (frame-root-window frame)))
            undelete-frame--deleted-frames))
-    (if (> (length undelete-frame--deleted-frames) 16)
+    (if (> (length undelete-frame--deleted-frames) undelete-frame-max)
         (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--save-deleted-frame -75)
-    (remove-hook 'delete-frame-functions
-                 #'undelete-frame--save-deleted-frame)
-    (setq undelete-frame--deleted-frames nil)))
+(add-hook 'delete-frame-functions #'undelete-frame--save-deleted-frame -75)
 
 (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.
+Without a prefix argument, undelete the most recently deleted frame.
+With a numerical prefix argument ARG between 1 and `undelete-frame-max',
+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)
-      (user-error "Undelete-Frame mode is disabled")
-    (if (consp arg)
-        (user-error "Missing deleted frame number argument")
-      (let* ((number (pcase arg ('nil 1) ('- -1) (_ arg)))
-             (frame-data (nth (1- number) undelete-frame--deleted-frames))
-             (graphic (display-graphic-p)))
-        (if (not (<= 1 number 16))
-            (user-error "%d is not a valid deleted frame number argument"
-                        number)
-          (if (not frame-data)
-              (user-error "No deleted frame with number %d" number)
-            (if (not (eq graphic (nth 0 frame-data)))
-                (user-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 frame-data undelete-frame--deleted-frames))
-              (let* ((default-frame-alist (nth 1 frame-data))
-                     (frame (make-frame)))
-                (window-state-put (nth 2 frame-data) (frame-root-window frame) 'safe)
-                (select-frame-set-input-focus frame)
-                frame))))))))
+  (if (consp arg)
+      (user-error "Missing deleted frame number argument")
+    (let* ((number (pcase arg ('nil 1) ('- -1) (_ arg)))
+           (frame-data (nth (1- number) undelete-frame--deleted-frames))
+           (graphic (display-graphic-p)))
+      (if (not (<= 1 number undelete-frame-max))
+          (user-error "%d is not a valid deleted frame number argument"
+                      number)
+        (if (not frame-data)
+            (user-error "No deleted frame with number %d" number)
+          (if (not (eq graphic (nth 0 frame-data)))
+              (user-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 frame-data undelete-frame--deleted-frames))
+            (let* ((default-frame-alist (nth 1 frame-data))
+                   (frame (make-frame)))
+              (window-state-put (nth 2 frame-data) (frame-root-window frame) 'safe)
+              (select-frame-set-input-focus frame)
+              frame)))))))
 \f
 ;;; Window dividers.
 (defgroup window-divider nil
diff --git a/lisp/menu-bar.el b/lisp/menu-bar.el
index b6dbf209ec..df55b3be89 100644
--- a/lisp/menu-bar.el
+++ b/lisp/menu-bar.el
@@ -108,15 +108,9 @@ menu-bar-file-menu
     (bindings--define-key menu [separator-tab]
       menu-bar-separator)
 
-    (bindings--define-key menu [undelete-frame-mode]
-      '(menu-item "Allow Undeleting Frames" undelete-frame-mode
-                  :help "Allow frames to be restored after deletion"
-                  :button (:toggle . undelete-frame-mode)))
-
     (bindings--define-key menu [undelete-last-deleted-frame]
       '(menu-item "Undelete Frame" undelete-frame
-                  :enable (and undelete-frame-mode
-                                (car undelete-frame--deleted-frames))
+                  :visible (car undelete-frame--deleted-frames)
                   :help "Undelete the most recently deleted frame"))
 
     ;; Don't use delete-frame as event name because that is a special
diff --git a/src/frame.c b/src/frame.c
index f94dff0a60..1d0dcf6278 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -2385,7 +2385,7 @@ DEFUN ("delete-frame", Fdelete_frame, Sdelete_frame, 0, 2, "",
        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
+When `undelete-frame-max' is more than 0, the most recently deleted
 frames can be undeleted with `undelete-frame', which see.
 
 A frame may not be deleted if its minibuffer serves as surrogate