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

[Eli Zaretskii]

> From: Alan Third <athird@googlemail.com>
> Date: Wed, 6 Sep 2017 09:53:54 +0100
> Cc: 28342@debbugs.gnu.org, "Charles A. Roelli" <charles@aurox.ch>,
>       James Nguyen <james@jojojames.com>
> 
> I can replicate on ms windows using just:
> 
> (setq scroll-margin 5)
> (setq scroll-preserve-screen-position 1)
> (setq mouse-wheel-scroll-amount '(1))
> 
> Open a large file. Make sure you're at the start of the file
> 
> M-<
> 
> Scroll down with the mouse wheel and within a few clicks point jumps to the 
> middle of the screen. A few more
> and it jumps to the end of the file.

Even simpler: after evaluating the first 2 of the above 3 settings,
visit xdisp.c, then type this twice:

  C-u 1 C-v

After you type this the second time, Emacs will infloop.

This bug is already present in Emacs 25.1.

I will look into this when I have time.  Thanks for the recipe.