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

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: all_but_last@163.com,  37829@debbugs.gnu.org
> Date: Sun, 20 Oct 2019 13:43:58 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > And doing this the opposite way makes no sense to me, it's in effect
> > the same as removing the feature.
> 
> I don't understand.  Doing it the opposite way would be just as
> expressive.

The idea behind this feature was that most faces shall not be
extended, so doing it the opposite way would mean we need to change
the definitions of an unlimited number of faces, including those not
in core.

> > > 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.
> 
> You may quibble, but changing the look this radically is breaking the
> code for me.

I suggest to run with it for some time, you may change your mind.  It
happened to many of us.

> > 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?
> 
> Because that's the way Emacs has worked since forever: If you put a face
> on a newline, then it'll extend to the end of the line.

We considered that a side effect of the implementation.  Many other
applications don't do that.

> We do that because that's the way we wanted the display to look.  If we
> didn't want that, we didn't put the face on the newline.  

Others said the exact opposite: that they want to be able to do that
without having the face extended.  Also, the automatic extension in
Emacs 26 and before behaved inconsistently in GUI and text-mode
frames, and even between different attributes (color vs underline, for
example).