Builtin screensaver deliberately get disabled while gsd-power-manager is running. See bug 686339.

See functions "gsd_power_enable_screensaver_watchdog" and "disable_builtin_screensaver" in https://github.com/GNOME/gnome-settings-daemon/blob/master/plugins/power/gpm-common.c

See functions "gsd_power_manager_start" and "gsd_power_manager_stop" in https://github.com/GNOME/gnome-settings-daemon/blob/master/plugins/power/gsd-power-manager.c

Hardcoded XSetScreenSaver = 0 and hardcode DPMSSetTimeouts = 0 sounds a bit overkill. Those two commands are executed every two minutes.

I think an handle to disable this behaviour is require. Or at least, it should be enabled only when "Power -> Power Saving -> Blank screen" value is different than "Never" in the control center. Some thing like this : On blank screen value changed, if value == Never, disable "gsd_power_enable_screensaver_watchdog". If value != Never, enable gsd_power_enable_screensaver_watchdog.

Otherwise, it totally destroy a nice and functional feature and prevent any users to use it if they wish. Some good Display Manager depend on xset values and are unusable while gsd-power-manager is running.

I also think more trace should be added in the logs to help people diagnose whats happening because it's impossible to trace it right now. More transparency with this behaviour could be appreciated by the community.

A little warning message could be welcome in the control center to explain that if Blank Screen feature is enabled, xset is going to be disabled and some display manager won't work properly. The users could like to know that Blank screen should be set to never if they wish to use a different display manager than GDM.

I hope you agree with this,
Thank you for your time!