[ESS] When the R window opens, sometimes it is side-by-side with my .Rnw file. I want them one above the other. It seems unpredictable

[Christopher W. Ryan]

I'm using Vince Goulet's modified version of emacs 26.2 (build 1,
x86_64-w64-mingw32)  of 2019-04-13 with ESS, on Windows 10. I use Sweave
and LaTeX, so I have my R code in .Rnw files. When I execute my first
line of R code, an R buffer opens of course. I prefer it to be below my
Rnw buffer. Sometimes it is. But other times the R buffer opens to the
side. It's unpredictable, which is disconcerting; I have not been able
to discern a pattern.  How can I ensure that every time emacs/ESS opens
an R buffer, the emacs window is split top-and-bottom, rather than
side-by-side.

Thanks.

--Chris Ryan