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

[Eli Zaretskii]

> From: Markus Triska <triska@metalevel.at>
> Date: Thu, 10 Jan 2019 20:57:28 +0100
> 
>      1) inserts several (i.e, window-height) lines into the buffer
>      2) sets window point to p, which is point after (1)
>      3) WAITS FOR A KEY PRESS (please press a key at this point)
>      4) erases and restores the buffer contents
>      5) sets window point to p, the same point as in (2)
> 
> However, in many invocations (please try it repeatedly if necessary),
> the visible portion of the buffer unexpectedly differs between (3) and
> after the whole procedure.
> 
> In other words, when Emacs waits for a key press, a different portion of
> the buffer is visible compared to after you press a key, even though
> window-start is set to the same position in both cases.
> 
> Is there a way to reliably set the window-start in such situations?

I think this happens because you call set-window-start with last
argument non-nil.  Doing that tells the display engine that the
window-start point is just a suggestion, not a hard requirement.