RE: Emacs pretest: highlight line number in display-line-numbers-mode?

[Drew Adams <drew.adams@oracle.com>]

> > >   . using a different face makes redisplay slightly slower
> > >     (because it disallows some redisplay optimizations)
> >
> > Would those optimizations still be available if a
> > user chose the same face for both (but the faces
> > were different by default)?

?

I guess you are confirming that the optimizations you
spoke of are a red herring here: they would anyway be
available whenever someone customizes the current-line
face to have the same appearance as the other-lines face.
Or do you want to correct this guess?

> > If so, I'd suggest making the faces different by
> > default.
> 
> They are already different by default.  They just have
> the same attributes.

I think you understood perfectly well that I was
referring to a difference of _appearance_, not the fact
that there are two different face objects.  There are
two faces, but they have the same appearance by default.
That's the misdesign.

Please consider giving them a different appearance by
default.  That will help users see which is the current
line, and it will help users know that there are two
different faces, which they can customize.

   If you think it is important, add a doc mention that if
   the faces have the same attribute values then display
   performance can be better.

> > IOW, opt-in for the loss of a difference.
> 
> That's already so.

No.  If the appearance is the same by default then
you have to opt out of that default behavior to see
a difference.  Lack of a difference in appearance
is not opt-in currently; it is the default behavior.