bug#6671: moving point and scroll-conservatively

[Chong Yidong]

Juanma Barranquero <lekktu@gmail.com> writes:

> But the trick is making it fast enough, isn't? IIRC, the current
> sluggish performance and some other changes were brought in to cope
> with redisplay pausing for a while and then suddenly "jumping ahead"
> in the buffer because it wasn't fast enough to keep up with typematic
> repeat during scrolling (pre-Unicode2 merge it always was,
> post-Unicode2 scrolling line-by-line is jumpy the best of days,
> specially when moving up the beginning of the buffer).

I don't think the problem is keyboard-triggered commands changing the
buffer too fast between redisplays.  I've done a bit of tracing, and
redisplay is called once for every one, some times two, calls to
command-execute.  Certainly not the ten or twenty times that would cause
window scrolling to fall behind.

Instead, the recentering is caused by the situation I described in an
earlier email, where the action of fontification functions causes
redisplay to become confused.  I don't know whether there are other
causes of anomalous recentering, but that is the only one I have
encountered while experimenting with the code.