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

[Juri Linkov <juri@linkov.net>]

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>>> >> > I tried to use this, but undelete-frame-mode in the File menu
>>> >> > makes no sense: when you mistakenly deleted a frame, you want
>>> >> > to undelete it immediately, so you open the File menu, and
>>> >> > see the message "No way, you can't undelete the deleted frame,
>>> >> > because you were careless and not enabled a special mode".
>>> >> >
>>> >> > So the most useful case for this feature is to get the
>>> >> > accidentally deleted frame back, and it fails to do this.
>>> >> >
>>> >> > Instead, it allows undeleting 16 frames in a special mode.
>>> >> > Is there really a human that can delete 16 frames, and then
>>> >> > remember what was on the 16th frame back?
>>> >> >
>>> >> > Rereading this thread indicates that the only concern about
>>> >> > enabling this by default was the memory footprint for remembering
>>> >> > 16 frames.  OTOH, this feature is really useful for remembering
>>> >> > 1 frame.  So this is what should be enabled by default:
>>> >>
>>> >> It seems this is the right thing to do, so now pushed to master.
>>> >
>>> > I'm sorry, you cannot do that.  We discussed this at some length and
>>> > reached certain conclusions.  Then you come and in effect say those
>>> > considerations and discussions make no sense, and you know better?
>>> > Let's please respect our discussions and decisions more than that.
>>> > And if you want others to respect your opinions, please respect
>>> > theirs, even if you disagree.  The feature as installed allows you to
>>> > customize it to have that mode turned on by default, so you could
>>> > easily fix your problem by doing that.
>>> >
>>> > Specifically to your main argument: it is no different from deleting a
>>> > file: unless the user took steps to configure the system to allow
>>> > undeleting deleted files, deleted files are lost forever.  Moreover,
>>> > in the case of an Emacs frame, nothing of terrible importance is
>>> > actually lost: the buffers displayed in that frame are still there,
>>> > and restoring the deleted frame by hand shouldn't take more than a few
>>> > moments.
>>> >
>>> > So I reverted this changeset.  Please in the future don't make such
>>> > changes unilaterally.
>>>
>>> This is not true.  This is not a unilateral change.  I posted a patch,
>>> then waited for comments 3 days, and when no one commented this means
>>> that everyone agreed that it's a more reasonable change, then pushed to
>>> master.
>>
>> I guess 3 days is not enough, especially in this time of year.  My
>> rule of thumb is to wait at least a week, possibly two.
>>
>> But in any case, the amount of time you waited is not the main issue
>> here.  The main issue is that we decided to implement this the way we
>> did, and you were even part of that discussion.  It makes no sense to
>> undo all that because you suddenly don't like the results.
>
> After applying the patch to test whether it correctly restores the tab-bar,
> I discovered that the weirdest thing was added to the main menu.
>
> No other app has such unusual menu item in the File menu
> because this feature is useful for everyone.
> Just imagine trying to undelete a tab in a web browser,
> then failing to do this when the browser requires
> to enable a special mode to undelete tabs.  We can't afford
> to make Emacs more bizarre than it currently is, that
> would scare away users.
>
> Then I recalled that the main argument in the discussion
> against enabling this by default was the memory occupied
> by 16 deleted frames.
>
> So to simplify this I proposed a new option with the default value 1
> instead of the hard-coded number 16.  Then waited for 3 days
> that is standard practice.  And no comments means that everyone agreed
> with this improvement.

Here is the patch that fixes all these problems:


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diff --git a/doc/emacs/frames.texi b/doc/emacs/frames.texi
index ba58f70caf..c641b8ccb1 100644
--- b/doc/emacs/frames.texi
+++ a/doc/emacs/frames.texi
@@ -515,12 +515,14 @@
 @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 fdbfd9b1be..2e748ce7c5 100644
--- b/etc/NEWS
+++ a/etc/NEWS
@@ -287,11 +287,12 @@
 
 +++
 *** 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' when
+the new user option 'undelete-frame-max' has its default value 1.
+Without a prefix argument, undelete the most recently deleted frame.
+With a numerical prefix argument between 1 and '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 5926a4d748..599ffe591a 100644
--- b/lisp/frame.el
+++ a/lisp/frame.el
@@ -2529,6 +2529,13 @@
         (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")
+
 (eval-when-compile (require 'frameset))
 
 (defvar undelete-frame--deleted-frames nil
@@ -2536,7 +2543,7 @@
 
 (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."
+Only the `undelete-frame-max' most recently deleted frames are saved."
   (when (frame-live-p frame)
     (setq undelete-frame--deleted-frames
           (cons
@@ -2555,54 +2562,45 @@
                         (cons '(display . :never)
                               frameset-filter-alist))))
            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--handle-delete-frame -75)
-    (remove-hook 'delete-frame-functions
-                 #'undelete-frame--handle-delete-frame)
-    (setq undelete-frame--deleted-frames nil)))
+(add-hook 'after-init-hook
+          (lambda ()
+            (add-hook 'delete-frame-functions
+                      #'undelete-frame--handle-delete-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)))
-             (frames (frame-list))
-             (frameset (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 frameset)
-              (user-error "No deleted frame with number %d" number)
-            (if (not (eq graphic (car frameset)))
-                (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 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)))))))))
+  (if (consp arg)
+      (user-error "Missing deleted frame number argument")
+    (let* ((number (pcase arg ('nil 1) ('- -1) (_ arg)))
+           (frames (frame-list))
+           (frameset (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 frameset)
+            (user-error "No deleted frame with number %d" number)
+          (if (not (eq graphic (car frameset)))
+              (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 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
diff --git a/lisp/menu-bar.el b/lisp/menu-bar.el
index e5a070b24a..36cbd6a9c5 100644
--- b/lisp/menu-bar.el
+++ a/lisp/menu-bar.el
@@ -109,14 +109,9 @@
       (bindings--define-key menu [separator-tab]
         menu-bar-separator))
 
-    (bindings--define-key menu [enable-undelete-frame-mode]
-      '(menu-item "Enable Undeleting Frames" undelete-frame-mode
-                  :visible (null undelete-frame-mode)
-                  :help "Enable undeleting frames in this session"))
     (bindings--define-key menu [undelete-last-deleted-frame]
       '(menu-item "Undelete Frame" undelete-frame
-                  :visible (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 959f0c9c14..e5d74edc16 100644
--- b/src/frame.c
+++ a/src/frame.c
@@ -2385,7 +2385,7 @@
        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