It seems I currently do not have a means to force the display on at all times. My system is a laptop in a docking station with an external monitor on the DVI(HDMI1 according to Display Settings) port. The laptop monitor is disabled. I used the 'Power Manager' applet under Applications Menu -> Settings. On the 'On AC' tab I selected monitor then dragged both sliders to 'Never'(extreme left). After less than 25 minutes of inactivity, the external monitor was switched off anyway. Other power management features appear to work correctly. If I select the Actions tab and change the 'When laptop lid is closed' setting, Linux correctly does the specified action when I close the lid.
There is more power management than just xfce4-power-manager. Did you disable blanking both in xscreensaver and the xserver itself (e. g. with 'xset dpms off')?
I don't have xscreensaver installed. Initially I just used the GUI but based on your message I tried 'xset -dpms' and the monitor still turned off. According to xset, 'xset dpms off' isn't a valid command.
Does the screen also go blank when xfce4-power-manager is not running?
Nope. I just killed it and nothing happened.
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