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

[Eli Zaretskii]

> Date: Sat, 12 Jan 2019 20:42:04 +0000
> Cc: rudalics@gmx.at, triska@metalevel.at, 34038@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > Almost certainly the fact that the last window's line is not 100%
> > visible.
> 
> No, it's not that, or at least, not only that.  I see the scrolling on
> my Linux tty, where lines cannot be partially visible.

What do you mean by "you see the scrolling"?  The recipe as published
shows the results only once, at the end, so there's nothing to compare
that display with to say there was scrolling.

What am I missing here?