I don't know much about this, and am only reporting there because there is a bug open on Gentoo's tracker and the Gentoo devs have requested it be reported upstream.
It appears that Gentoo's Q&A tool is reporting that there's a bashism in
> * QA Notice: shell script appears to use non-POSIX feature(s): > * possible bashism in /etc/init.d/xpra line 31 (should be 'b = a'): > * if [ "$?" == "0" ];then
I think this is browser/xpra/trunk/src/service/xpra|xpra/trunk/src/service/xpra in your VCS - appears to be line 31
Applied in r16698, will be backported eventually.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1624