bug#66769: 30.0.50; pixel-scroll-precision-mode and scroll-margin regression

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: aaronjensen@gmail.com,  66769@debbugs.gnu.org
> Date: Sat, 28 Oct 2023 15:35:13 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > What is the "target point" in the above text? target for what?
> 
> The position where point will be moved after the window start and
> vscroll are adjusted, which is such that redisplay will not recenter or
> otherwise undermine the scrolling operation.

And what are the problems in computing this target point in the
particular case described here?

> > Why not use the too-slow posn-at-point, but only in this case?
> 
> Because with that, precision scrolling slows down to a crawl.

Even if it's done "only in this case"?  It should slow down only this
case, no?

And what exactly is the crucial difference between "this case" and the
other cases, where scrolling works correctly?