Steps to reproduce:

1. Open attached project
2. Open scene "TestScene"
3. Run it
4. In game view, move mouse cursor towards the bottom of the game window. Once near the edge, slowly move mouse down and notice that Y value outputted in console is negative but mouse is still in the game view

Additional steps:
5. Open MousePositionDebug.cs script and add line to output Screen.height (when I've tested this it was 536)
6. Now Run scene and this time move mouse towards the top of the window
7. Notice that once mouse pointer is at the top, Y value is 531 => This means that Y value is shifted by -5

Note: UI elements are unclickable if mouse is positioned over them but Y value is negative.

Reproduced with: 5.4.0a5, 5.4.0b12
Not reproducible: 5.3.4p1, 5.4.0a4
Regression introduced in 5.4.0a5