bug#33929: 26.1; Stop vertical window split from switching to horizontal

[Eli Zaretskii]

> Date: Sun, 30 Dec 2018 11:09:17 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> I wanted to see more of the rightmost part of *Backtrace*, without
> scrolling, so I enlarged the frame with the mouse, extending it to the
> right.  At some point the split changed to horizontal, putting the
> *Backtrace* window at the right instead of the bottom.  SURPRISE!?!
> 
> This of course made things even worse, as the *Backtrace* window was
> then even narrower than it was before extending the frame to the right.
> 
> More importantly, I *didn't ask* Emacs to change the window-split
> direction.  This should not happen (by default), IMHO.
> 
> Sure, I can compensate, e.g. by using `C-x 5 2' or by scrolling.  But a
> user shouldn't have to compensate for Emacs planting obstacles in her
> way suddenly, out of the blue.

You can control this with split-height-threshold and
split-width-threshold.