Re: Emacs canvas support

[Eli Zaretskii <eliz@gnu.org>]

> From: David Engster <deng@randomsample.de>
> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel@gnu.org
> Date: Wed, 29 Apr 2020 18:14:55 +0200
> 
> IMHO, What would be much more exciting, is if you could draw directly
> over normal text. This would enable a whole new set of
> possibilities. For instance, I always liked visual diff tools like
> 'Meld' or 'Code Compare', but I don't see how you could do this with
> Emacs. Or things like proper indentation guides, or widgets inside text
> for code folding. Yes, those can be done in Emacs today, but it is very
> difficult to get right and often not really satisfactory (weird
> interactions between packages, too slow, etc.).

AFAIU, Meld seems to draw the graphics _between_ windows, and only
highlights the source lines with background colors.  I think we can do
that if we use an extra window in-between the two being compared, and
put images in that extra window to show the graphical description of
the changes between the two versions.

Drawing over normal text, if we don't want to redesign the entire
display engine, needs some new kind of "display element" ( a sibling
to "character", "image", "stretch", etc.), one that doesn't
necessarily have any effect on the metrics of the screen lines it is
drawn upon.  I'm not sure I have a clear idea about what features such
a drawing will need to support, but it could be possible to add such
an element with not too much effort.  Would someone want to come up
with a reasonable list of requirements for such a feature?