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

[Eli Zaretskii]

> 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.