For a long time I have been locking a session by light-locker manually but can't find the clear way to setup locking by timeout. I installed xscreensaver for this purpose and it worked somehow for me. When the time to lock came xscreensaver locked the session properly. When I needed to lock it by my hands I could but when I unlocked it back I was required to enter my password twice.
Since I got lubuntu 17.10 installed I uninstalled xscreensaver because I meant it is no need to run screensaver just to see blank screen. Currently I still can to lock the session manually by shortcut or appropriate menu item (utilizing light-locker as before) but can't find out any way to get xfce4-power-manager/light-locker pair to lock the session by timeout. I get blank screen by timeout but my session remains unlocked.
I google the issue but found a number advices how to disable the locking but not to enable.