Thanks for the details . . . in my case, running XFCE w/lightdm, was at issue, but then changing over to xdm did not “work” until I ran your suggested systemctl commands.
Now that I did that I could use update-alternatives to change back to lightdm and that “works” now.
But somewhere in that mix up video play has again been messed with. YT works, but other sites do not.