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

[Eli Zaretskii]

> Date: Sun, 13 Jan 2019 14:40:28 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: 34038@debbugs.gnu.org
> 
>  > There's no bug here: Emacs by default avoids starting a window's
>  > display at EOB.  Scroll-conservatively overrides that.
> 
> AFAICT a first redisplay tries to obey a forced 'window-start' at EOB.
> A subsequent redisplay_preserve_echo_area, however, has forgotten the
> force_start and recenters the window.

Yes.

> Which means that force_start is pretty ephemeral.

It's ephemeral if it leaves the window in a state which will make
subsequent redisplay unhappy.  Which is what this recipe triggers.