bug#61496: 30.0.50; Default value of icon-title-format

[Eli Zaretskii <eliz@gnu.org>]

> From: Óscar Fuentes <ofv@wanadoo.es>
> Date: Tue, 14 Feb 2023 01:02:04 +0100
> 
> 
> AFAIK icon-title-format was broken since long time ago (possibly for
> several major releases) and it was fixed recently.
> 
> I propose that its default value should be nil, and interpret that value
> as "same as frame-title-format". That's why:
> 
> 1. The case of having an specific frame title when it is iconified seems
>    to me as way less frequent than expecting that the frame keeps the
>    same title.
> 
> 2. After upgrading to Emacs 29, users that set frame-title-format will
>    see how frames change their title when iconified.
> 
> 3. There are mechanisms for applying settings or performing actions
>    depending on the title of a frame (KDE Window Rules and scripts based
>    on xdotool, for instance.) For keeping those mechanisms on a working
>    state with Emacs 29, the user must ensure that either it keeps
>    icon-title-format synced with frame-title-format and/or his scripts
>    must be adapted. Without that, any config that depends on the
>    content's of the frame title will be broken (unless the user already
>    set icon-title-format, but it would be surprising if he did, as that
>    setting had no effect until now.)
> 
> Having a working icon-title-format, in practice, is a new feature, so
> defining a new default for it shouldn't have any impact. Certainly, it
> will not have a visible effect compared to recent Emacs releases.
> However, keeping its current default value may cause confusion and
> breakage for any user that sets frame-title-format on his config.
> 
> The required change in code is simple enough:
> 
> src/xdisp.c | 3 ++-
> 
> modified   src/xdisp.c
> @@ -13424,7 +13424,8 @@ gui_consider_frame_title (Lisp_Object frame)
>  
>        Fselect_window (f->selected_window, Qt);
>        set_buffer_internal_1 (XBUFFER (XWINDOW (f->selected_window)->contents));
> -      fmt = FRAME_ICONIFIED_P (f) ? Vicon_title_format : Vframe_title_format;
> +      fmt = FRAME_ICONIFIED_P (f) && !NILP (Vicon_title_format) ?
> +	Vicon_title_format : Vframe_title_format;
>  
>        mode_line_target = MODE_LINE_TITLE;
>        title_start = MODE_LINE_NOPROP_LEN (0);
> 
> ... plus a trivial doc change on its DEFVAR_LISP and initialization.
> 
> If this proposal is acceptable, it should be applied to Emacs 29, to
> avoid putting ourselves on a similar scenario when Emacs 30 is released.

I agree with adding a feature that will allow to keep
icon-title-format and frame-title-format identical.  However:

  . I don't want to make that the default: it's too late for such
    changes on the emacs-29 branch.
  . I suggest that the "special" value which means "use
    frame-title-format" will be t, not nil, so that users who want it
    set it explicitly, not by some omission.  (Code-wise, this is not
    important, since the code treats both nil and t the same: it
    produces nothing.  So using t or nil is backward-compatible, in
    the sense that older Emacsen will not crash.)

I understand that you think the more radical change you propose is for
the better, and "no one can possibly suffer of be against it", but I
think you are not very objective, being a victim of the problem, and
it is quite possible that someone out there does want/expect the
default value to be a string.  Thus, I think we should leave the
change of the default value for some future release.

So: okay for changing xdisp.c, just using EQ (Qt, ...), not NILP, but
please don't change the default value of icon-title-format.

Also, this change needs a suitable NEWS entry and appropriate changes
for the doc string and the ELisp manual.

Thanks.