Log in as root in the Xfce environment is disabled by default. When I tried to google how to change this behaviour, I got lots of results, but useless, because though dozens of people had posted that same question in dozens of forums, they all got the same kind of reply: “Log in as root in graphics mode is dangerous. You shouldn’t do it, and certainly I won’t tell you how to.” Bullshit. That’s a hypocritical reply, as coming from someone who refuse to help others under the weak grounds that “it’s dangerous”. I can’t help thinking that those “gurus” are simply enjoying their power at knowing something they don’t want to share.
Fortunately, I was lucky to find out the answer by myself. Here’s how you can do it (I’m talking about the Linux Mint Xfce and the gdm3 session manager):
Open a terminal and, as root, edit the file /etc/pam.d/gdm3. Comment out the line that says: “auth required pam_succeed_if.so user != root quiet success”.
There you are! That simple. Now you can log in as root in an Xfce session.