xflock4 does not turn off screen due to lack of sleep
Submitted by Chris Bainbridge
Assigned to Simon Steinbeiß
As noted at http://www.shallowsky.com/linux/x-screen-blanking.html calls to "xset dpms" need to be prefixed with "sleep 1":
will blank the screen (or activate the screensaver program, if you're using one) after a delay of one second. You need the delay because X gets a little confused about the order of events; it will blank the screen but immediate un-blank it, thinking the activity of your typing the command happened recently enough to come out of screensaver mode. sleep 1; xset dpms force off will turn the screen OFF after a delay of one second. You can also use standby, suspend, or on instead of off.
Downstream bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=776674