I was having this issue.
killing xscreensaver, and running
xscreensaver -verbose
gave me a bunch of:
xscreensaver: 01:23:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:24:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:24:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:25:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:25:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:26:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:26:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:27:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:27:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:28:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:28:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:29:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:29:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:30:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:30:58: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
xscreensaver: 01:31:28: ClientMessage DEACTIVATE received while inactive: resetting idle timer.
I then started looking for stuff to kill. one thing was xfce4-power-manager, but that didn't do the trick.
There was no vlc window open, but I found a task running with
$ ps aux | grep vlc
poleguy 5568 0.1 0.0 1983844 12476 ? Sl Jan04 27:43 /usr/bin/vlc --started-from-file
it wouldn't kill normally so I did a
kill -kill 5568
and the DEACTIVATE messages went away! Not easy to track down, so I hope this helps someone trying to track down similar issues.
My monitors will sleep tonight!