Steps to reproduce:
1. Open attached "Temp.zip" project
2. Have a stopwatch ready
3. Press play on the editor and start the stopwatch at the same time
4. Cmd+Tab between two other applications a couple of times on fullscreen and let it sit for a while (up to 2 minutes should be enough)
5. Go back to Unity and observe the counter having fewer seconds than the stopwatch

Expected outcome: If Unity runs in the background it should do it reliably
Actual outcome: It counts seconds up to a certain point and then stops

Only reproduced on MacOS
Reproduced on: 5.5.4p1, 5.6.2p1, 2017.1.0f3, 2017.2.0b2