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: Sat, 12 Jan 2019 14:25:57 +0100
> 
> In this snippet, I try to restore the exact same configuration that you
> see when "Step 1 ..." is displayed, i.e., window-start is set so that
> point is on the very first line, which is blank. When you press a key at
> this point, then a few more lines of text are inserted. Then, please
> press another key when "Step 2 ..." is displayed.
> 
> What I observe after these forms is that point is unexpectedly placed
> vertically centered in the window instead of at the topmost line.

Maybe I didn't invoke the recipe exactly as you do, but here I get the
result you expect, I think: point is on the topmost line.