bug#44232: global-display-fill-column-indicator-mode vs. readonly buffers

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Mon, 26 Oct 2020 12:17:07 +0100
> Cc: 44232@debbugs.gnu.org
> 
> 積丹尼 Dan Jacobson <jidanni@jidanni.org> writes:
> 
> > (A workaround might be for
> >     fill-column is a variable defined in ‘C source code’.
> >     Its value is 70
> > to be turned off for Dired, Gnus, etc. buffers.)
> 
> That's true.  `special-mode' could set it to nil, and then everything
> that inherits from that would have no fill column.
> 
> Does anybody see any disadvantages to that?

This is an opt-in feature.  People who don't like it can solve that
very easily: by never enabling this mode.

As for the specific suggestion: it sounds somewhat kludgey to me.  Why
not take the Emacsy way: add a defcustom that lists major modes whose
users are unlikely to want this mode turned on, and let the default
value include the modes you think belong to that group.  (It should be
a defcustom because I can easily envision someone who may wish to see
this indicator even in those modes, perhaps to use it for some purpose
other than filling, e.g. to find filenames longer than some limit in
Dired.)