How to reproduce:
1.Create a new Unity project
2. Make Mac Standalone build (make sure that Resizable window is enabled)
3. Press the green full-screen button in the toolbar and wait until app enters full-screen
4. Press the green button again
--Window will exit full-screen and immediately returns back to full-screen

Expected result: Mac player should exit and enter full-screen window after one clicking it
Actual result: Mac player window returns from full-screen and then immediately comes back

Fixed in: 2019.1.0a2

Regression first introduced in: 2017.2.0a4
Reproducible with: 2017.2.0a4, 2017.2.0b1, 2017.2.0b4, 2017.2.0b10, 2017.2.0f2, 2017.3.0b3
Not reproducible with: 5.5.5f1, 5.6.3p4, 2017.1.1p2, 2017.1.1f2, 2017.2.0a3