FYI: On Ubuntu 18.04 there is a dialog showing that the game doesn't react with options to wait and force quit it. That dialog immedialely gets hidden behind the game window but keeps capturing the input so the game appears frozen. After switching to the dialog and selecting wait the game runs perfectly well. =)
Yep, seems to be an unfortunate combination of Gnome Shell's latest versions being a bit overzealous with this popup (I get the same for almost every heavier application I start), and Unity's splash screen feature. If it stays as an issue in the long run we can always just ditch Unity's splash screen stuff and build out own, but for now I'd actually wait and see if Gnome gets it back to a bit better balance again...