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

[Gerd Möllmann]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Manuel Giraud <manuel@ledu-giraud.fr>
>> Cc: alan@idiocy.org,  67794@debbugs.gnu.org
>> Date: Wed, 13 Dec 2023 17:50:08 +0100
>> 
>> BTW, do you know why mouse-highlight is this way?  Is it because it
>> was an afterthought of the display engine or is it because of
>> performance issues or…?
>
> How would you design it instead?
>
> I'm not saying that what we have is the only possible design, but I'm
> curious what alternative design ideas could be used.
>
> 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.

Maybe things could be done differently today, since event handling is no
longer done in SIGIO handlers.