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

[Juri Linkov]

>> So all concerns raised by Eli are addressed now
>> that make possible to enable it by default
>> as was discussed on emacs-devel:
>
> Once again, please do NOT enable this by default, I didn't agree to
> that.  This feature should remain disabled by default for some time,
> until we have enough information to discuss whether it should be
> turned on by default.  As discussed and agreed during its development.

But after the recent discussion it was agreed to enable it by default
after fixing raised concerns.  Everything is fixed now.
Should we restart the same discussion again?

>>  @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
>
> "One of the recently deleted frames" sounds like Emacs arbitrarily
> decides which one.  Which is not  what you wanted to say, surely.

This was copied from old text: "undelete one of the 16 most recently deleted 
frames".

>> +(defcustom undelete-frame-max 1
>> +  "Maximum number of deleted frames before oldest are thrown away."
>
> This doesn't say how to disable the feature.

This feature can be disabled by customizing it to 0.

>>      (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)
>
> I think Gregory didn't like the disappearing item, and preferred it to
> be visible at all times, even if disabled.

The invisible item makes the already overly long menu shorter,
especially for users who don't use frames.