Alt-Tab process switcher fails in VNC session

When xfce4 is run in the VNC session Alt-Tab shows up but doesn't disappear.

In order to reproduce install TightVNC, set up XFCE4 as desktop environment in VNC sessions, start the VNC session, connect to it, press Alt-Tab.

OpenGL doesn't work by default in VNC sessions, otherwise everything works. Desktop environments should work fine in VNC sessions.

OS: FreeBSD 12.2 xfce-4.16.0