bug#459: Zero-length overlays, overlay keymaps, and `overlays-at'

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  459@debbugs.gnu.org,
>   t.s.cubitt.98@cantab.net
> Date: Mon, 19 Jul 2021 20:23:39 +0200
> 
> Stefan Monnier <monnier@iro.umontreal.ca> writes:
> 
> > I vote for a) with or without the requirement of a new optional arg.
> > The reason I vote for a) rather than b) is because AFAICT `overlays-at`
> > does not pay currently attention to from/rear advance properties, so it
> > would be weird to do that just for empty overlays.
> 
> That's an excellent point.
> 
> I'll have a look at doing a) tomorrow, then, unless we (in the mean
> time) decide to just include these overlays without adding a new
> parameter.

No, please don't make that the default behavior.  It makes no sense to
change the default behavior of such a veteran API for the benefit of a
rare use case.  The gains are very small, and don't justify the risks.
An optional argument gets us the best of both worlds, so it's a clear
winner.