Re: Tick Reduction

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: larsi@gnus.org,  emacs-devel@gnu.org
> Date: Fri, 19 Nov 2021 16:09:44 -0500
> 
> > The display code only assumes the current glyph matrix is up-to-date
> > if a set of very conservative tests succeeds.  In the other cases, it
> > doesn't use the contents of the current glyph matrix.
> 
> So far it indeed doesn't use the old glyph matrix, but that matrix is
> still around and (for the lines that preceded the currently processed
> text) should still be up-to-date.
> So we could conceivably use that.

IME, that road is full of pitfalls and mines.  The current display
code stays away of that for a good reason.

> > And if this is not enough, let me remind you that the display engine
> > also includes a set of functions that "emulate" redisplay, and those
> > cannot use the glyph matrices at all, because they many times are used
> > for text that is not displayed at all.
> 
> Yes, we may have to declare that for functions that "emulate" the
> redisplay internally, the resulting horizontal position info might not
> always be quite right (might not reflect what you'll see on the glass)
> for text tat uses the new alignment functionality :-(

That will break quite a few features, I'm afraid.  Including redisplay
cycles themselves, which sometimes use this emulation to decide how to
proceed or which buffer position to use as window-start.  But also
vertical-motion, for example, and stuff that depends on it, like
line-move-visual.