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

[Po Lu]

Aaron Jensen <aaronjensen@gmail.com> writes:

> Commit 1da4fca0647ebf1d5d6f12817301a17661560810 caused a regression of
> bug#52231
>
> The repro is the same:
>
> (progn (setq scroll-margin 4)
>        (pixel-scroll-precision-mode))
>
> And scroll down a buffer with mouse wheel.
>
> The buffer does not scroll properly, it jumps back unless you scroll
> fast enough.

Hmm, I'm not certain what the solution to this should be.

For images to scroll properly, the "target point" must be derived from
whether the point is visible after scrolling, instead of outside a set
number of rows from the window start or end.  Yet the latter information
is mandatory if the scroll margin is to be taken into account, and no
function supplies both besides posn-at-point, which is much too slow.

The immediate remedy is to restore the old code when scroll-margin is in
effect and document the consequent incapacity to scroll over large
images as an unfortunate corollary.  Is that acceptable by you?