Comment 1 for bug 1469700

I found:
http://ubuntuforums.org/showthread.php?t=1475925

"xtrlock"

xtrlock is a very minimal X display lock program, which uses nothing
except the Xlib library. It doesn't obscure the screen, it is
completely idle while the display is locked and you don't type at it,
and it doesn't do funny things to the X access control lists.

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I will try to test this and figure out how to lock the screen now ...