bug#6671: moving point and scroll-conservatively

[Eli Zaretskii]

> From: Chong Yidong <cyd@stupidchicken.com>
> Cc: Eli Zaretskii <eliz@gnu.org>, 6671@debbugs.gnu.org
> Date: Thu, 24 Mar 2011 22:09:28 -0400
> 
> 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.

If you can prevent this confusion in important use cases, please do.

But I really think that we need to be able to position point at the
first/last line of the window regardless of any "confusion".  This is
what users who set scroll-conservatively want.  They don't care how
"confused" can the display engine become.  And I think you cannot
prevent confusion altogether, because with input that comes in at high
enough rate, Emacs will eventually fall behind and will be unable to
use try_scrolling.  With today's code, if try_scrolling fails, we will
always recenter.