As I can see from the screenshot on the official website, Grilla uses one single display interface to load all the other menus that are activated. It means that when another function is activated, its corresponding window will be placed above with the possibility to move it around. However, you have the possibility to perform auto-window selection using the View action in the menubar.
To find more about this software, access the contact page on the official website and use the contact details to contact the developer.
Contact page: http://www.tromino.it/contacts.htm