Suppose I am watching Youtube the turns completely dark & when I move the mouse I see the lock screen & I need type my user password to unlock.
I have disabled power management. I also checked the screensaver settings but screensaver is already disabled.
if you got the power icon right down, right click you see presentation mode;
when it is enabled, the screenlocker does not activate for sure…
Done ! I need time to test this. I will reply after 1 hour.
It worked. Thanks.
xfce4-screenlocker works basicly like any other xdg-screensaver compatible like lightlocker etc… Xset turns in then it locks…depend on configs… if you dont want to load the xfce4-screenlocker then you can always disable it from autostart ^^ 
but then is xset stil there but presentation mode avoid it goes to black.