XFCE un- lock screen not working (XFCE4, lightdm)

In Xubuntu 18.x and Ubuntu XFCE 19.04, 19.10 (XFCE4, lightdm) I experience the un-lock screen does not show up. The screen remains locked!

Actually typing the password into the invisible lock sceen OPENS the lock!!!

That means the password request is working but does not show up. This is the reason why there are no error messages.

"light-locker-settings" explicitly tells me XFCE is controlling the "energy" settings. This works.

And there is no cover window over the GUI when it is "locked". The window in front of the main GUI does not show up (information not covered) and password entry does not show up.

QUESTION: How can I make the password request visible again??

Solution 1:

light-locker is buggy, see e.g. Bug #1832960, Bug #1855753

In 19.10 you can use xfce4-screensaver for locking, instead of light-locker. In 18.04 (and in 19.10) you could use xscreensaver instead of light-locker. You have to install the screensaver first. Make sure only the one you use is autostarted during login in "Session and Startup" dialog.

You could use which locker you want with Xubuntu 19.10, see here for instructions. As an administrator you could use any locker with an older release, too, by putting your version of xflock4 script in /usr/local/bin. (Otherwise the default version at /usr/bin/ is used.)

Solution 2:

The availability of the XFCE lock screen(s) DEPENDS on the window manager!

• lightdm is NOT capable according to an error message first seen in U20.10.

• gdm3 IS capable to enable the lock screen.

So the issue is partly resolved: It DEPENDS on the window manager.