I had to do this last week, I was thinking of ways to get my splash screen to load up the main screen whilst still displaying a throbber. in the end I think i was over complicating things I Simply called my splash widget to come up first on my main level, and then once I clicked a button it would display a throbber with a 2 second delay before removing widget. since scene was loading straight after widget was displayed, by the time the user clicked the button the scene was ready for use. it worked for me since loading time was no more than 3 seconds, depends how long your scene takes to load?