bug#28342: 26.0.50; (OSX) Jumpy Scrolling using Trackpad

[Eli Zaretskii <eliz@gnu.org>]

> From: Nick Helm <nick@tenpoint.co.nz>
> Cc: Eli Zaretskii <eliz@gnu.org>
> Date: Thu, 07 Sep 2017 13:58:45 +1200
> 
> The problem might lie in window_scroll_pixel_based in window.c. A small
> tweak seems to help:
> 
> --- src/window.c	2017-09-07 13:16:29.000000000 +1200
> +++ src/window.c	2017-09-07 13:10:04.000000000 +1200
> @@ -4835,7 +4835,7 @@
>  	 have the `scroll-command' property.  This avoids the
>  	 possibility of point becoming "stuck" on a tall line when
>  	 scrolling by one line.  */
> -      if (window_scroll_pixel_based_preserve_y < 0
> +      if (window_scroll_pixel_based_preserve_y <= 0
>  	  || !SYMBOLP (KVAR (current_kboard, Vlast_command))
>  	  || NILP (Fget (KVAR (current_kboard, Vlast_command), Qscroll_command)))
>  	{

Thanks, but this change only fixes the problem when you start
scrolling with point on the first line in the window.  If point is on
2nd, 3rd, or 4th line (more generally, any line inside the
scroll-margin), the bug will still be there.

I installed on master a more general fix, which I hope solves this
problem.