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

[Drew Adams]

> > This change should be reverted as soon as possible.
> 
> It needs not be reverted, I think.  We just need to make the
> priority of the region overlay higher than any other overlay.

1. I meant that the change to the behavior of the region not
appearing "on top of" other highlighting (except isearch) needs
to be reverted (undone).  IOW, bug #15899 is a regression.  It
is the behavior that needs to be fixed, regardless of how that
is done.

2. Region highlighting should *not* be higher priority than
all other overlays.  It should not be higher than isearch
highlighting, for instance.  There might be other exceptions
too; dunno.  See my previous mail.

3. It's not clear that the reported "bug" was in fact a bug,
rather than the intended Emacs behavior.  Or if it was, it is
not clear just what the bug was.  If it was thought to be a
bug that other highlighting was in some cases overruled
by region highlighting, then that was not, IMO, a bug.

4. If it was in fact a bug, it's not clear why the fix for it
needed to involve changing region highlighting to use an
overlay.  Not clear to me anyway.

My suggestion is to first revert the code change and then
discuss what the bug is that it was intended to fix.  If
there is really a bug that needs fixing, then let's please
try to find some other, non-shotgun fix for it.

There should be no need to change the longstanding behavior
of the Emacs region just because someone's highlighting does
not show through.