bug#54156: [External] : bug#54156: 28.0.91; set-face-attribute with a nil FRAME doesn't change the default

[Eli Zaretskii <eliz@gnu.org>]

> From: Drew Adams <drew.adams@oracle.com>
> CC: "damien@cassou.me" <damien@cassou.me>,
>         "54156@debbugs.gnu.org"
> 	<54156@debbugs.gnu.org>
> Date: Sat, 26 Feb 2022 16:17:19 +0000
> 
> I'm not sure I understand this from Eli, however:
> 
>   The correct way to do what Damien wants (AFAIU) is this:
> 
>    (set-face-attribute 'region nil :background 'unspecified)
>    (set-face-attribute 'region t :background 'unspecified)
> 
>   That is, one must explicitly call set-face-attribute
>   with FRAME = t (as well as nil), and pass 'unspecified'
>                ^^^^^^^^^^^^^^^^^^
>   (NOT nil!) as the value.  Maybe we should document that,
>   although it is a obscure and unusual thing to do.
> 
> My impression is that it's enough to do this:
> 
> (set-face-attribute 'region nil :background 'unspecified)
> 
> I'm probably not testing/checking this correctly,
> but it seems to me that both the selected frame
> and new (future) frames are affected by using
> nil for the frame (and `unspecified' for the
> face attribute value).

I'm not sure this isn't the result of the particular implementation we
have, so I prefer to tell people to call with FRAME = t explicitly.
After all, this is a rare use case.

> Eli, you say "This is well-documented: a color
> must be a string or 'unspecified'."  Still, it
> wouldn't hurt to add something to that effect in
> the doc of `set-face-attribute' - and say what
> the effect is of using nil (for the face color)
> - it's tolerated (no error) but it doesn't have
> the effect of `unspecified'.

We generally don't advertise compatibility shims for obsolete
features, because we want people to stop using them.