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

[Drew Adams <drew.adams@oracle.com>]

I don't split windows often, but sometimes I do when I'm debugging
something from `emacs -Q'.

I was debugging something from `emacs -Q', with *scratch* in the top
window and `*Backtrace* in the bottom window (no other windows in the
frame).

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.

Dunno just when this changed, or why, but it seems wrong, to me.  Please
consider backtracking on such a "DWIM" surprise.  I don't care about
this problem for myself, as I don't encounter it with my setup.  But a
priori, and naively, I'd think that this would be bothersome for other
users - hence this bug report.  If you disagree then just close the bug.
Thx.


In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''