Window auto-resizing not working in FreeBSD guest in virtualbox

I installed virtualbox-ose-additions as described here but resolution auto-resizing isn't working. I searched around and from what I've read configuring Xorg isn't necessary anymore, it should detect the virtualbox driver automatically. What am I doing wrong? If it matters, the host is Windows 10.
Make sure X is actually using the vboxvideo driver and not something else. Also make sure to load VBoxClient when your session starts. And make sure xrandr(1) is installed.