This is a follow-up of gtk+ bug http://bugzilla.gnome.org/show_bug.cgi?id=572387 :
when using non randr 1.2 compatible video driver (old i810 for instance), calling XRRGetScreenResourcesCurrent or XRRGetScreenResources returns no monitor unless XRRGetScreenSizeRange is called before (you won't notice if you use xrandr to query randr info, because it calls XRRGetScreenSizeRange first).
This is with X server 18.104.22.1683 and doesn't happen with xrandr 1.2 compatible drivers.
Mass closure: This bug has been untouched for more than six years, and is not obviously still valid. Please file a new report if you continue to experience issues with a current server.