Game hangs after Unity splash screen - user must click to regain window focus

I've had a tester experience an issue with my game running on OSX (built in 2017.4.5f1).

I don't have a Mac so I've been unable to test it myself.

The issue is:

• User starts the game via Steam.
• Game opens full-screen and shows the Unity splash screen.
• Once the splash screen finishes, the first scene appears for 1 frame.
• The splash screen background then appears full-screen (without the Unity logo this time).
• Game hangs in this state.
• Once the user clicks anywhere on the game window, the splash screen background fades out into the first scene and it resumes as normal.

This only occurs is another app is open in the background (which it pretty much always will be, because my game is launched from Steam). If you minimise all apps while the game is starting to launch then it works fine.

Can anyone help me fix this please?