Re: Scrolling commands and skipping redisplay, was: Re: emacs rendering comparisson between emacs23 and emacs26.3

[Dmitry Gutov]

On 20.04.2020 18:01, Eli Zaretskii wrote:

> > > It's not a "merge" and not an optimization, it's how Emacs was
> > > designed.  It only enters redisplay when it's idle.  If you invoke
> > > commands one after another quickly enough, more than one command will
> > > be executed before the next redisplay cycle.
> >
> > The fact that Emacs's behavior can depend on when redisplay happens, and
> > the user cannot reliably control it, is problematic IMHO.
>
> Please be specific: what Emacs's behavior can depend on when redisplay
> happens, apart from whether something was or wasn't updated on
> display?

The window's scroll position.

Again: you (setq scroll-conservatively 1) and lean on C-n for a little 
while. Depending on whether Emacs manages to redisplay after every 
command invocation, the point will, or will not, scroll to the middle of 
the window. It could do that several times as well.

> > > > At the beginning of each command, we can save the value of point. If, at
> > > > the end, we're still in the same buffer, and the new value of point
> > > > fails the scroll-conservatively check, count the lines between the
> > > > previous value of point and the new one. If that number is <=
> > > > scroll-conservatively, then scroll "conservatively" anyway.
> > >
> > > And if not, then what?
> >
> > Then give up and move point to the middle of the window.
>
> But that's again "not good" according to your opinion, isn't it?

Why wouldn't my suggestion be good enough?

> Or
> maybe you assume this will solve most of the cases when we currently
> recenter under scroll-conservatively < 101?

All important ones, yes.

> > > Anyway, scrolling commands don't normally move point, they move the
> > > window-start.  Point then moves because redisplay brings it into
> > > view.  So I don't think your algorithm will work, at least not in the
> > > usual case.
> >
> > I'm a lot less worried about scrolling "conservatively" when the user
> > actually invokes the scrolling commands.
>
> Ahem... the thread's title begins with "scrolling commands",so I
> thought we were still discussing that?

The amount of times I have mentioned 'C-n' might have persuaded otherwise.

> > > What you describe might be relevant to "scrolling" that happens if you
> > > lean on C-n, not on C-v.
> >
> > C-n, or C-M-e, or C-s. These are indeed scenarios when the current
> > behavior can be most surprising.
>
> Is it?  Just out of habit, I suppose.  What Emacs does make a lot of
> sense: it shows you the optimal amount of context.  By contrast,
> scroll-conservatively shows you only one side of that context, and
> usually the wrong side.

Are we going to argue whether scroll-conservatively is useful? I hope 
not. And if didn't assign value to that "optimal amount of context" 
thing you mention, I'd be arguing in favor of making 
scroll-conservatively a boolean instead.