I can confirm the bug on Xubuntu 18.04. My graphics card is an nVidia GeForce GTX 570 (GF110).
I tried with a fresh Ubuntu 18.04 (and not Xubuntu) and there's no bug.
So I think the issue is related to lightdm, which is used on Xubuntu and not Ubuntu.
I can confirm the bug on Xubuntu 18.04.
My graphics card is an nVidia GeForce GTX 570 (GF110).
I tried with a fresh Ubuntu 18.04 (and not Xubuntu) and there's no bug.
So I think the issue is related to lightdm, which is used on Xubuntu and not Ubuntu.