bug#34038: 26.1; set-window-start sometimes fails to set window start

[Eli Zaretskii]

> Date: Sat, 12 Jan 2019 18:22:03 +0000
> From: Alan Mackenzie <acm@muc.de>
> Cc: Markus Triska <triska@metalevel.at>, 34038@debbugs.gnu.org
> 
> The question remains, what is causing the unexpected scrolling in
> the OP's scenario?

Almost certainly the fact that the last window's line is not 100%
visible.