From some research, I think this is a bug, but I'm not sure. It seems that the VBoxClient program, which is needed to actually start some of the additions, will not function if run from a non-root account. I can confirm with a fresh 5.2 install/update that VBoxClient (--clipboard, --display, or any of the other options) will not perform the desired function. Using -d to see what's going on produces
Running VBoxClient with root permissions works fine. I tried adding the user to the appropriate user groups (come to think of it, it's almost easier to emerge gentoo from a Stage 1 tarball than it is to actually enable guest additions!
) and this was no help. Our brothers in Debian suggest the problem is that VBoxClient uses /dev/vBoxUser, which non-root doesn't have write access to. I checked and noticed that the user and group for this file is "root". Changing the group to vboxguest with a user already assigned to vboxguest enabled VBoxClient to start from a non-root account. The problem is that this file is apparently recreated or something on each reboot, as the owner/group would be reset to "root" on each reboot.
Can anyone else confirm that 1) there's no magic step I'm missing (and I've fiddled with rc-update script thingies and /etc/conf.d/modules whatchamacallits that made no difference) or 2) that VBoxClient is intended to be able to be run without root access?
I'm not even going to ask about why compiz doesn't want to start in Virtualbox; I'm guessing/hoping that's just 733 things that need to be added to xorg.conf.
Oh, and I also tried uninstalling the guest additions and emerging the more recent guest additions from portage, but that didn't help... other than providing a stress test for my CPU.