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

[Eli Zaretskii]

> From: Markus Triska <triska@metalevel.at>
> Cc: Eli Zaretskii <eliz@gnu.org>,  34038@debbugs.gnu.org
> Date: Fri, 11 Jan 2019 18:45:53 +0100
> 
> >From this, I get the impression that a pending redisplay may interfere
> with reliably setting the window-start even if window-point is set!

If you call set-window-start with last argument non-nil, redisplay is
under no obligation to honor your window-start.  It tries to use it,
but if it doesn't fit, for whatever reason, it disregards it.

So as along as you call set-window-start this way, you cannot rely on
the window-start point you set to be honored.  It might be, except
when it won't be.