bug#20510: 25.0.50; Underscore hidden by underline

[Eli Zaretskii <eliz@gnu.org>]

> From: Fabrice Niessen <fni-news@pirilampo.org>
> Cc: 20510@debbugs.gnu.org
> Date: Wed, 06 May 2015 21:00:16 +0200
> 
> I mean: can't the hl-line-mode code be written in a way that the
> underline goes in the background of the character, that is that every
> pixel of every character gets written "over" the underline line?

hl-line-mode cannot control that, and nothing on the Lisp level can.

When hl-line-mode puts its overlay on a line, the characters of that
line have the hl-line face merged with their face, with the result
that each character of that line will have the underline face in the
color you specify.  That's all Lisp can do; then along comes the
display engine and redraws the characters in this new merged face.
The C code which draws characters is written so that we first draw the
background, then the characters, then the underline.

So what you are actually asking is that the order of drawing the
underline (and also overline, strike-through, etc.) is changed so that
these attributes are drawn _before_ the characters.  That should be
possible by changing the order in the C code, but I have no idea what
that will do in other situations.  It's probably not TRT at least when
we draw images and other similar stuff instead of characters.

Perhaps someone who knows more than I do about GUI display back-end
could tell if this is a safe change.