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

[Eli Zaretskii]

> From: Óscar Fuentes <ofv@wanadoo.es>
> Cc: 61496@debbugs.gnu.org
> Date: Tue, 14 Feb 2023 14:35:56 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > 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.
> 
> The whole point of this change is to not break user's setup when they
> upgrade to 29. If the user must fix his config, he may as well set
> icon-title-format and be done with it.
> 
> OTOH, there is no possible breakage by changing the default.

(Famous last words.)

Seriously, though: I can easily concoct a situation where this change
will break someone's setup: imagine that someone's customizations
expect icon-title-format to be a string.  Boom!

> >   . 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.
> 
> The purpose of changing the default is based partly on the idea that,
> very likely, the user will want to keep icon-title-format in sync with
> frame-title-format. So not having to do anything to achieve that is a
> feature, not a bug.

We are mis-communicating.  My point is that the value nil means "don't
display this".  E.g., try setting mode-line-format or
frame-title-format to nil, and observe the effect.

So setting it to nil could mean the user really doesn't want the title
be displayed.  Thus, usurping the nil value to mean "use
frame-title-format instead" could be a backward-incompatible change.
By contrast, I don't expect anyone to set the value to t, because
doing so would require intimate knowledge of the display-engine
internals (which in this case treat nil and t the same).  So using t
for this special purpose is better from backward-compatibility POV.
It is also more logical, since the user _does_ want a title, just the
"default" one.

> > and it is quite possible that someone out there does want/expect
> > the default value to be a string.
> 
> I have no idea how this can be.

Please believe me and my gray hair: with Emacs, anything that is just
possible is usually done by someone out there.

> > Thus, I think we should leave the
> > change of the default value for some future release.
> 
> ... which, as I explained, will require another round of fixes on user's
> config to avoid the same problem when 30 is released.

No, it won't.  If the user sets icon-title-format to t, that will work
with Emacs 29, and will still work if and when we make t the default
value in future versions.

> If this is your final decision, it's better to discard the whole
> proposal, as it gives little benefit.

I disagree.  I think it will allow you and others in your situation to
have the problem solved, while not risking breaking someone's
unrelated customizations.  I don't understand the urge to have it
solved automagically, even for those who are affected by the changes
in Emacs 29.