The XSync semantics mandate that alarms already expired will not
fire until the counter is reset and the alarm triggered again, so
clients traditionally called get_idle_time() first to see if they
should install the alarm.
This is inherently racy, as by the time the call is handled by
mutter and the reply received the idle time could be different.
Instead, if we see that the watch would have fired in the past,
fire it immediately.

This is a behavior change, but it's a compatible one, as all legacy
clients are calling get_idle_time() first, and it was perfectly
possible for the idle time counter to trigger the alarm right
after the get_idle_time() call.

https://bugzilla.gnome.org/show_bug.cgi?id=707302