Is there a trick to using "debug mode" (seamlessless mode)? it seems to do nothing

In both 4.0 and 4.1, shutting down a virtual machine, going into qube settings, and checkmarking “debug”, then clicking apply & ok, then starting the VM via launching a xterm or firefox does not seem to activate seamlessless (single window) mode. Staring it with “start qube” does not seem to change anything.

using qvm-prefs for the qube shows:
debug - True

Does “debug mode” (seamlessless mode) work for debian qubes for anyone else?
(have not tried fedora yet)

Maybe “debug” needs to be set before the AppVM starts for the first time?