"/usr/bin/VBoxClient --vmsvga" causes black screen inside VirtualBox host

This concerns virtualbox-guest-x11 6.1.32-dfsg-1build1 in Ubuntu jammy.

I am running the Xubuntu (Xfce) desktop inside of a VirtualBox host (5.2.42-dfsg-0~ubuntu1.18.04.1) on Ubuntu bionic.

The system boots fine, and I get to the lightdm login prompt without issue. Shortly after logging in, however, the entire (guest) screen turns black.

The desktop appears to be running normally, I can start X applications from a virtual terminal, I can even confirm with xev(1) that mouse movements are being recognized within the black screen. However, nothing I do gets anything to display, other than killing the session from a VT and returning to lightdm.

I have tracked down the issue to the invocation of "/usr/bin/VBoxClient --vmsvga" in /etc/X11/Xsession.d/98vboxadd-xclient . If I comment out that line, then the session appears normally. If I re-enable it, the black screen returns. Heck, if I run that in a terminal in a working session, the black screen returns. Killing the VBoxClient process at fault does not restore normal video.