I have noticed that
echo $DISPLAY on Ubuntu 18.04 LTS prints
:1 The same command on Ubuntu 16.04 prints
:0 Why it has been changed ?
For security reasons, and Wayland porting reasons, [GNOME] now launch the X server and Wayland server within the user's session, instead of starting one as root.
The way that we do this is that we launch two X servers, one for the gdm greeter session, and for the session user.
Ubuntu Desktop 18.04 supports running a Wayland session, even though it is not the default.