'sudo id -G <username>' vs 'id -G' issue

[Ralf Mardorf]

>>> newgrp    
>it should work for my purpose.

Oops,

no, it doesn't. It requires to run newgrp each time a new terminal is
used and after running newgrp _launching apps fails_.
                              ======================

ubuntu-mate at ubuntu-mate:~$ pluma
ubuntu-mate at ubuntu-mate:~$ firefox
ubuntu-mate at ubuntu-mate:~$ id | cut -d\  -f2
gid=999(ubuntu-mate)
ubuntu-mate at ubuntu-mate:~$ newgrp - alibaba
ubuntu-mate at ubuntu-mate:~$ id | cut -d\  -f2
gid=1001(alibaba)
ubuntu-mate at ubuntu-mate:~$ pluma
Failed to connect to Mir: Failed to connect to server socket: No such file or directory
Unable to init server: Could not connect: Connection refused

(pluma:29782): Gtk-WARNING **: cannot open display: 
ubuntu-mate at ubuntu-mate:~$ firefox
Error: GDK_BACKEND does not match available displays

Regards,
Ralf