bug#67794: 30.0.50; mouse-face is not respected on SVG images

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Manuel Giraud <manuel@ledu-giraud.fr>,  alan@idiocy.org,
>   67794@debbugs.gnu.org
> Date: Wed, 13 Dec 2023 18:57:13 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Btw, AFAIU the person who at the time decided how this will work is
> > here: it's Gerd (CC'ed).  Maybe he can share his memories about why he
> > ended up with this design.
> 
> IIRC, this way of mouse-highlighting stems from Emacs 19, at least in
> principle. At that time, X event handling was done from a SIGIO signal
> handler. Being called from a signal handler made it impossible to use
> anything from the event handling code which wasn't reentrant. And there
> was not much reentrant code in Emacs, in particular not Lisp, but also
> most other stuff.

That's what I thought.

Still, given that the response to mouse movement should be very fast,
and the glyphs are already there, just with the wrong colors, I think
a design that just reuses the glyphs for redrawing a portion of the
screen sounds kinda...natural.