OK, this wasn't a good example as:
sleep 2 ; gdbus call --session --dest org.gnome.Mutter.DisplayConfig --object-path /org/gnome/Mutter/DisplayConfig --method org.freedesktop.DBus.Properties.Set org.gnome.Mutter.DisplayConfig PowerSaveMode '<3>'
works.

gnome-settings-daemon doesn't seem to be calling for mutter to blank the screen when doing Win+L.

$ gdbus introspect --session --dest org.gnome.SessionManager --object-path /org/gnome/SessionManager | grep IsActive
      readonly b SessionIsActive = false;

That's why gnome-settings-daemon isn't poking at the screen.

$ loginctl 
   SESSION        UID USER             SEAT            
         1       1000 hadess           seat0           
        c1         42 gdm              seat0           

2 sessions listed.

$ loginctl show-session 1 | grep -i active
Active=yes
State=active

Created attachment 298753 [details] [review]
Fix SessionIsActive property

GsmSystemd emits its cold-plug notify::active signal out of
init(), giving GsmManager no chance to connect its signal handler
in time to catch the initial emission. And even if we did,
the GsmManager handler can only operate once the skeleton object
exists.
Therefore, do our own cold-plug after creating the skeleton.

This is fallout from the GDBus port.

Comment on attachment 298753 [details] [review]
Fix SessionIsActive property

Tested and works for me.

Attachment 298753 [details] pushed as cf40833 - Fix SessionIsActive property

Created attachment 298894 [details] [review]
manager: don't connect to systemd until exported on bus

We now properly resync our active state after being exported
over the bus, but we still have a signal connection set up
prematurely.  If it did get called, it would lead to scary
messages on the console.

This commit moves the signal connection down to the same place
we test our active state at startup.

Attachment 298894 [details] pushed as cd84c50 - manager: don't connect to systemd until exported on bus