It used to be the case that you could specify --with-Xdummy --with-Xdummy_wrapper when running setup.py build to force selection of the Xdummy wrapper rather than Xvfb without having the Xorg detection stuff kick in. Subsequently, running a python setup.py --skip-build install (without --with-Xdummy --with-Xdummy_wrapper) would install an xpra.conf file which used Xdummy, as expected.
Currently, unless --with-Xdummy --with-Xdummy_wrapper is specified both with build and install, the Xorg detection stuff kicks in during install, and a xpra.conf using Xvfb is installed.
Logically, I expect xpra.conf to be created during build, and whatever is present during install is just installed if we're using --skip-build. I'm not sure when this crept in.
out of time, and this stuff is always harder than it looks - rescheduling
see also #977
Too hard to fix for little gain.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/908