bug#15899: 24.3.50; regression: `region' overlay is lower priority than default

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 16 Nov 2013 12:25:36 +0200
> From: Dmitry Gutov <dgutov@yandex.ru>
> CC: monnier@iro.umontreal.ca, 15899@debbugs.gnu.org
> 
> `easy-kill' depends on region highlighting working in a certain way.

What way is that?  Before region highlighting was reimplemented as an
overlay, it had a fixed "priority" that couldn't be controlled or
worked around.  So what exactly did easy-kill expect from that
behavior?

> It has a command `easy-mark' which selects some unit of text around 
> point. And it uses a dedicated overlay to mark the place where point was 
> before the command was called, in color.

But the region always has point on one of its ends, so both the places
where point was and where it is are clearly visible when the region is
active.  So why is there a need for the easy-mark to be visible in
that situation (which is transient and therefore short-lived)?

> So that overlay needs to have higher priority than region. No need to 
> make region highlighting invisible.

The part of the region that overlaps the easy-mark overlay will not be
visible, if the region's priority is lower.  Or did you mean something
else?

> >> If the region overlay will have a high but finite and documented
> >> priority, that would be much better.
> >
> > Which will start an "overlay priority arms race", something I loathe.
> 
> I don't think so. The region overlay priority won't change, even if 
> people decide to shoot themselves in the foot and raise priorities of 
> overlays inappropriately.

That's not the race I had in mind.  What I had in mind was users
complaining about their favorite overlay-based features being obscured
by the region, lobbying the maintainers to increase the priorities of
those overlays above the region (and possibly also above the easy-mark
overlay), or increase the priority of the region overlay; then other
users complaining about the effects of that, and so on and so forth ad
nauseam.  How can you even assume that the "region overlay priority
won't change", given the possibility and enough pressure from users?
Once out of the bottle, this genie cannot be easily put back.