If "Always on top" is enabled, the XCenter will always remain
on top of all windows on the system. This is a "really-always-on-top" setting
and will affect the entire system.
Hint: If you don't like this setting, assign an object hotkey to the
XCenter on its "Object" page. If you then press that object hotkey and the
XCenter is already open, it will be resurfaced to the top of all windows.