According to this post: https://bugzilla.gnome.org/show_bug.cgi?id=759161#c4, X11 has the notions of "transient for group", and while it's an ICCCM violation, it's commonly used and documented that a window manager would treat a window with transient_for set to None to transient for all windows of its group.
If that's the case, we need to find an example to test and handle None in a special way.
From this rather old message https://mail.gnome.org/archives/wm-spec-list/2003-December/msg00049.html: Assuming DIALOG based on transiency is a legacy guess; metacity only does it if the type hint is unset.
So I don't think we should bother with this unless we actually find an application that misbehaves.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1140