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

[Alexander Miller]

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.

The bug can be reproduced as follows:

1) emacs -q
2) Create enough newlines to scroll down
3) (setq show-trailing-whitespace t scroll-margin 10 
scroll-conservatively 101)
4) Go to the top of the buffer and scroll down with C-n
5) When scrolling the cursor will first move one line less than the
margin away from the window bottom.
6) Press C-n again and the margin is used correclty, resulting in the
cursor visually jumping back a line.
7) The end result is that scrolling downward this way is a jittery mess
with the cursor constantly jumping back and forth.

In GNU Emacs 26.0.50 (build 1, x86_64-unknown-linux-gnu, GTK+ Version 
3.22.8)
of 2017-02-19 built on a-laptop
Repository revision: 938426d1ca0930b859c3e37b26513f5d74761284
Windowing system distributor 'The X.Org Foundation