At the moment, this flag can be set on a window but we don't do anything with it - we should pass it to the client via the window properties and the client should then toggle the fullscreen flag as requested.
Pointer: wm-spec _NET_WM_STATE
(somewhat related to #263)
works for me in r3414
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/264