One of my xfce4 desktops is a VM running inside Virtual Box. For whatever reason(s), the 3D functionality no longer works in the emulator.

This wouldn't have bothered me, because I do not use the system for anything 3D, but, unfortunately, various aspects of the desktop itself broke. For example, the panel and window manager are still coming up, but most of the panel add-ons (like weather or xkb) no longer start.

The mousy-menu is visible and pops up, but cannot start most of the items -- the message (captured in ~/.xsession-errors) is always the same, complaining about lack of 3D support.

I'm fairly certain, this is not a problem of the individual components, but is somewhere in the code they share.

FWIW, lyx -- a Qt5 application -- would not start either.

Likewise, gvim was refusing to start too -- until I recompiled it to use Athena widgets instead of GTK2.