bug#25792: 26.0.50; Scroll-margin working incorrectly when scrolling down if show-trailing-whitespace is t

[Eli Zaretskii]

> Date: Sun, 19 Feb 2017 19:20:40 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: alexanderm@web.de, 25792@debbugs.gnu.org
> 
> > From: npostavs@users.sourceforge.net
> > Date: Sun, 19 Feb 2017 11:59:22 -0500
> > Cc: 25792@debbugs.gnu.org
> > 
> > > Only scrolling down is affected. The issue first appeared when I
> > > recompiled emacs roughly 1 week ago and has persisted over multiple
> > > recompilations on 2 different laptops (both running Antergos & Xfce), so
> > > my guess is that it's an issue in the current emacs master.
> > 
> > This seems to be caused by [1: b9be4c14e8], it only happens when the
> > window has partial lines (i.e., (zerop (cadr (cl-floor
> > (window-screen-lines)))) is non-nil).  Not sure how
> > show-trailing-whitespace affects this, but it does.
> 
> My guess would be that show-trailing-whitespace causes an additional
> redisplay cycle, due to its post-command-hook.

If you invoke trace-redisplay and then run the recipe, you will see
that there are two kinds of redisplay cycles: one that calls
try_scrolling, the other that doesn't.  So there are 2 different paths
through redisplay, and they don't behave the same under these
conditions.

HTH