Re: Scrolling commands and skipping redisplay

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Apr 11, 2020 at 16:45:02 +0300, Eli Zaretskii wrote:
> > Date: Sat, 11 Apr 2020 13:21:26 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: rudalics@gmx.at, rrandresf@gmail.com, rms@gnu.org, emacs-devel@gnu.org
> > 
> > My personal view: As defaults, I think (i)a and (ii)a are bad, due
> > to the long intervals Emacs appears to have hung in.  Of (i)b and (ii)b,
> > I think (i)b is better, since it avoids the "twinkling" of (ii)b.
> > 
> > I dislike (iii)a,b because of the appearance of non-fontified screens,
> > but particularly because the last screen gets fontified in situ after
> > first appearing unfontified.  I think the non-fontification looks poor
> > to a user, even though the scrolling is smooth.
> > 
> > If it were up to me to chose a default setting, I would prefer (i)b,
> > closely followed by (ii)b.

> Disclosure: I didn't implement any of these options.

:-)

> I dislike (i)b and (ii)b because they are jerky, and because when I
> stop leaning on C-v, I have a lot of screen to watch scrolling, until
> Emacs starts executing new commands.  I guess my auto-repeat rate is
> higher than Alan's. or maybe it's because I'm trying this in a GUI
> frame, not a TTY frame.

It sounds like your time for scrolling a screen, even without
fontification, is close to your auto-repeat time.  My repeat rate is 30
characters/second.

> My favorite (if one needs to alleviate the default, where Emacs hangs
> -- I never actually lean on C-v for prolonged times, except for
> testing this very issue) is (iii)a, because it gives me an
> uninterrupted scrolling and immediate response once I release C-v.

Each one of our sets of (dis)likes is reasonable.  That's a strong
argument for retaining both jit-lock deferred fontification and
fast-but-imprecise-scrolling.

What seems not so good is the difficulty a user, on experiencing the
problem of 15 second hangs, would have in finding both of the above
solutions.  What do you think of adding a paragraph like the following
after the description of f-b-i-scrolling on the page "Scrolling" in the
Emacs manual:

    As an alternative to setting `fast-but-imprecise-scrolling' you
    might prefer to enable jit-lock deferred fontification (See Font
    Lock).  To do this, customize `jit-lock-defer-time' to a small
    positive number such as 0.1.  This gives you less jerky scrolling,
    but the buffer contents on any scroll operation into a fresh portion
    of the buffer will at first be unfontified,

?

-- 
Alan Mackenzie (Nuremberg, Germany).