bug#37829: 27.0.50; Overlay behaviour changed without documentation.

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Zhu Zihao <all_but_last@163.com>,  37829@debbugs.gnu.org
> Date: Sun, 20 Oct 2019 13:16:08 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Crystal ball says it's because the mode-line face doesn't have the
> > :extend attribute by default.  If so, this change _is_ in NEWS and in
> > the ELisp manual.
> 
> Hasn't there been sufficient fallout now from this change that we should
> consider doing the :extend stuff the opposite way?

No, I don't think so.  All the complaints until now were about Diff
mode and related modes, and we made the relevant faces extend a few
hours ago.  At least in one case, this change allowed to _remove_ some
kludgey code (in info.el).

And doing this the opposite way makes no sense to me, it's in effect
the same as removing the feature.

> This change breaks a lot of out-of-tree code.

No, it doesn't break any code.  It changes how display looks in some
cases, so people are surprised at first.

For this particular bug report, why would someone expect the overlay's
color to extend to the end of the line instead of affecting only the
text that the overlay covers?