

$ groups
xavier666 root
Sorry what? As what user are you executing all these ‘groups’ commands? Unless Ubuntu does things significantly differently from Arch and Debian, there’s something very fishy going on here. The “normal” user should not be in the root group, and root should not be in the normal user’s group.
Have you done other things beside the “fix” you mentioned?
That “fix” from your op, btw, looks totally valid to me.
And how did you use to start i3?