Hi and thanks for the answer.
I didn't try it but have you checked this one?
In interface property, there is option 'show interface permanently'.
Yes, I've ticked this option.
If my dialog interface is on top of the interface list, it always overlays the other interfaces no matter, if 'show interface permanently' is enable or disabled.
If my dialog interface is below the other interfaces in the interface list, it overlays the other interfaces, when 'show interface permanently' is enabled, otherwise it doesn't.