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

[Eli Zaretskii <eliz@gnu.org>]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: Alan Third <alan@idiocy.org>,  67794@debbugs.gnu.org
> Date: Tue, 12 Dec 2023 15:07:13 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Basically, SVG images specify their own background color, and the
> > Emacs display cannot override that, since the image is generated by
> > librsvg.  So to change the background color, we wrap the SVG in
> > another SVG, see svg_load_image.  This way, the SVG spec submitted to
> > librsvg specifies different colors according to what Emacs wants.  And
> > we only do that for colors that come from 'face' properties.
> 
> This means that when you do a 'C-s foobar' and that the SVG is correctly
> displayed with the isearch face (foreground and background), such an
> embedded SVG was created on the fly?  If so, I think we should do the
> same for mouse-face too because this SVG generation is very fast!

Not sure "fast" is fast enough, since unlike face properties,
mouse-highlight is highly dynamic, and creating a new image each time
(as I think this would mean) is something we want.  Let's wait for
Alan to chime in with his insights.

> >> (note: I have tried to track this down but debugging Emacs display is
> >> *hard*.  I have read and tried the "Debugging Emacs redisplay problems"
> >> section of etc/DEBUG with no luck so far.)
> >
> > The mouse highlight is implemented in note_mouse_highlight and its
> > subroutines.  If you already discovered that, then what were the
> > difficulties you faced in understanding what happens?
> 
> Yes, I used to put a break point on show_mouse_face but then diving down
> to x_draw_glyph_string it is hard to "see" what is going on here.

We are drawing a series of glyphs of the same type which share the
same face/mouse-face.  For SVG, it would probably mean we are drawing
that single SVG (which is a single glyph), since series of images are
usually rare.

> As it was working with face such as isearch, my new strategy was to
> find out how it worked here but now I don't know where/how to break
> to the correct place.

show_mouse_face calls draw_row_with_mouse_face, which calls
draw_glyphs, which builds a "glyph string" and then calls the frame's
draw_glyph_string method, which on X is x_draw_glyph_string.  The
latter eventually calls x_draw_image_glyph_string.  HTH.