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

[Manuel Giraud via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

[...]

>> Ok and so my next question is: Is mouse highlight handled completely
>> outside the « regular » display engine?  If so, I will never ever see a
>> lookup_image call with a face_id == MOUSE_FACE_ID, right?
>
> Right.
>
> mouse-highlight works differently than the "regular" display: it
> redraws already drawn portions of the screen by (re-)drawing the same
> glyphs with different colors.

Ok.  And it works with a pixmap because it uses its mask.  But for SVG,
we have a fully rasterized image without mask.  Thank you for clarifying
it.

> By contrast, lookup_image is called when a 'display' property is found
> during _generation_ of those glyphs, before showing them for the first
> time.

Yes but the glyph generation is not done for every face under the sun,
right?  So this could be done "dynamically"?

If so, what would be a right place to have a call to lookup_image into
mouse-highlight?  Is it "too late" for glyphs generation?
-- 
Manuel Giraud