I believe that all applications that support this do so by exporting their menu structures over DBus at all times, not just when they are focused. For context,

I believe that all applications that support this do so by exporting their menu structures over DBus at all times, not just when they are focused. For context, a launcher could provide access to all application menus, but prioritize results based on which applications were most recently accessed. Or just keep it simple and go with only the most recently focused one.