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 20:08:23 +0100
> 
> > If so then does (setq scroll-conservatively 101) fix it?
> 
> Yes, with scroll-conservatively set to 101, I get the intended effect,
> i.e., the same position of window point as after Step 1 (79) also after
> the whole form is evaluated! This is what I expect also with the default
> settings, i.e., scroll-conservatively set to 0, for this snippet.

Most probably, given the size of your default font, the last line of
the window is not fully visible, and so Emacs recenters point to make
its line visible.  With scroll-conservatively it intentionally avoids
doing that because that's what scroll-conservatively is all about.
But that option sometimes causes expensive calculations during
redisplay, and many people don't like the effect even when it causes
no slow-down, so it is not the default.