bug#63271: 29.0.90; broken mouse-face

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: Eli Zaretskii <eliz@gnu.org>,  juri@linkov.net,  63271@debbugs.gnu.org
> Date: Tue, 09 May 2023 15:12:06 +0200
> 
> On Tue, 09 May 2023 20:52:24 +0800 Po Lu <luangruo@yahoo.com> wrote:
> 
> > What if you change the font driver in use to something else, like X?
> > i.e.
> >
> >   (set-frame-parameter (selected-frame) 'font-backend "x")
> 
> With that the highlighting problem in fundamental-mode vanishes: all
> problematic strings show mouse-highlighting. (FTR, with the "x"
> font-backend, the default face here is displayed with adobe-courier and
> variable-pitch face is displayed with adobe-helvetica.)

Does changing the font backend also changes the font used for the
variable-pitch face?  If it does, then perhaps you could force Emacs
to use the same font by customizing the variable-pitch face?  Since we
already know that the font somehow affects this issue, we need to try
to use the same font with different backends, to be sure it's the
backend that counts, not the font.