Reproduction steps:
1) Open "ConfinedMouseRepro" project
2) Build for standalone (make sure that "Default" scene is included)
3) Run built project in windowed mode
4) Try to move mouse out of the window in top side of a game window

Expected results:
Mouse cursor should stay in game window bounds, where game is happening.

Actual results:
Mouse cursor leaves bounds and can be clicked on game window's top panel.

Note:
This is reported because player may accidentally click on buttons in top right corner (watch attached gif)