Repro steps:
1. Open user attached project
2. Build and Run the project: File -> Build and Run
3. Run the build in Full Screen
4. Refocus the window by hitting Alt+Tab twice
5. Move the cursor out of the game window

Expected: The cursor is locked to the center of the game window
Actual: The cursor can be moved outside the game window

Reproducible with: 2018.1.0a1, 2018.1.6f1
Not reproducible with: 2017.1.4p2, 2017.2.3p1, 2017.4.6f1, 2018.2.0a1, 2018.2.0b9, 2018.3.0a2

Notes:
- This only happens when "Visible in background" is unchecked in the Player settings
- Two screens required to see the issue