"Gnome-screensaver was created in 2005 by people who thought it was a better idea to rewrite all of XScreenSaver from scratch instead of sending me patches for the changes they desired in XScreenSaver itself.
[...more vitriol for Mate, Cinnamon and Unity...]
The only way to securely lock your screen under X11 is to turn off gnome-screensaver, mate-screensaver, cinnamon-screensaver, etc. and use XScreenSaver instead."