Tested on OpenBSD 6.3 and 6.4
cwm(1) is my favorite window manager for X11. It has a tiling mode, so I don't have to rearrange windows manually.
cwm(1) is in OpenBSD base.
$ echo 'exec cwm' >> ~/.xsession
Here is my .cwmrc. Quite often I keep just two windows open. On the left side: tmux in xterm(1). On the right side: Firefox.