>> I didn't mean to keep actual frames around.  I thought about keeping
>> only the same data that is used by `clone-frame` to make a new frame
>> identical to the original frame.  In case of `undelete-frame` this data
>> can be used to make a frame identical to the deleted frame.
>> I.e. a lightweight version of framesets, that avoids loading frameset.el.
>> This mostly means that after deleting the frame, only frame parameters
>> are kept from garbage collection, that later can be reused when making
>> a new frame on undeletion.
>
> Ah, got it, thanks.  Then fully agreed.  I was heading in the same
> direction when I suggested to try and use as little as possible of
> `frameset.el` and move the resulting code to `frame.el`.

While copying some code from `clone-frame` to `undelete-frame`,
I noticed that `clone-frame` doesn't work on non-GTK builds,
because window-id frame parameters keep frame-specific X-windows,
so `clone-frame` is broken in emacs-28.

Eli, is it ok to push the first patch to the release branch?