Setting Cursor.lockState to Locked works normally at first, but after alt-tabbing to another window and back to the Unity editor (or player) the mouse becomes unlocked. It remains invisible while in the game window but can move outside of it.

To reproduce:

1. Download attached project "CursorLockBug.zip" and open in Unity
2. Open "SampleScene" scene
3. Enter Play mode
4. Move the mouse and observe that mouse is not visible
5. Alt+Tab to switch to another window
6. Alt+Tab to switch back to the Unity window
7. Move the mouse and observe that mouse becomes visible outside Game window

Notes:
- This issue appears both on Windows and OSX
- Workaround: Setting every frame and enabling "run in background" appears to work around the issue

Reproduced on Unity 2018.1.0a1, 2018.1.6f1, 2018.2.0b9 and 2018.3.0a2
Not reproduced on Unity 2017.4.6f1
Regression on Unity 2018.1.0a1