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

[Eli Zaretskii]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: alan@idiocy.org,  gerd.moellmann@gmail.com,  67794@debbugs.gnu.org
> Date: Wed, 20 Dec 2023 09:47:48 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> So with "ps", I measured the RSS (resident set, here in KB).  I start
> from "emacs -Q" with this patch.  I used the following code to test the
> feature (the SVG used here is 580x580 pixels so produces a quite large
> image on display):
> --8<---------------cut here---------------start------------->8---
> (defun +insert-stuff ()
>   (interactive)
>   (let ((str "foobar"))
>     (add-text-properties 0 1
>                        (list 'display
>                              (find-image
>                               '((:type xpm :file "close.xpm" :ascent center))
>                               t))
>                        str)
>     (add-text-properties 3 4
>                        (list 'display
>                              (find-image
>                               '((:type svg :file 
> "/usr/local/share/qgis/svg/tourist/tourist_fountain.svg"))
>                               t))
>                        str)
>     (add-text-properties 0 6
>                        (list 'mouse-face '(:background "red" :foreground 
> "blue")) str)
>     (insert str)))
> --8<---------------cut here---------------end--------------->8---
> 
> Here are the results:
> 
> Right after "emacs -Q": rss = 44688
> After 'M-x +insert-stuff': rss = 54292
> After first hover on text: rss = 54464
> After repeated hover on/off text: rss = 54496

Does it get bumped each hover-over, or does it stabilize after a few?

> > Btw, does this only work for SVG, and if librsvg is later than some
> > recent version?  Or does this work with any image?  If the former,
> > this code should be executed only conditionally, under those
> > conditions under which it really changes the background color of an
> > image.
> 
> You're right: it does only work with SVG.  XPM are already handled with
> mask.  I have tested and it does not work with PNG with transparent
> background: don't know why.  Maybe instead of guard it with a "only for
> SVG" test, we should resolve the issue for transparent PNG at a later
> time.

It would be a shame to waste cycles on PNG images as long as we don't
solve this problem, so I'd prefer to enable this only for SVG and
leave a FIXME comment about PNG.

> I'm using librsvg 2.57 and I could not easily test it with an older
> version.

Alan, is the trick with wrapping SVG with another one supposed to work
with all versions of librsvg?  I see some version conditions in
svg_load_image, but I'm not sure I understand the consequences.
Specifically, what does this code do:

  #if LIBRSVG_CHECK_VERSION (2, 48, 0)
    rsvg_handle_set_stylesheet (rsvg_handle, (guint8 *)css, strlen (css), NULL);
  #endif

It is only used for librsvg 2.48 and later.

And for librsvg older than 2.32, it looks like we don't use the
wrapped SVG at all?