Re: relative line numbers and folding: how to make they play along?

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Thu, 14 Jul 2016 17:03:07 -0400
> 
> >> Actually, I don't think there needs to be flickering if the first step
> >> ("perform redisplay of window") just computes the new matrices without
> >> performing any drawing.
> > Since the display engine computes the number of each screen line as it
> > lays them out, I don't understand why would 2 phases be needed.  I'm
> > probably missing something.
> 
> If we bake it into the redisplay code, we can indeed do it "on the fly",
> but if we want this feature to be implemented in Elisp it seems a lot
> more tricky to avoid the 2 passes.

But this sub-thread started with you talking about baking it into
redisplay:

> I think to implement relative-visual-linum-mode efficiently, we'd need
> help from the display engine.  E.g.:
> - First perform redisplay of the window.
> - then, go through the window, visual-line by visual-line
>   and add something in the margin.
> - then update the margin part of the matrices.

These two passes are described in terms of what redisplay does, they
are not visible to Lisp: we don't return to the command loop after
each window's redisplay, and the glyph matrices are not exposed to
Lisp, either.

So now I'm confused about what you are suggesting.