Steps to reproduce:
1. Open "MouseClickTest.zip" attached project
2. Enter play mode
3. Left Click and hold the button
4. Drag mouse outside of game view
5. While still holding left mouse, click right or middle mouse
6. Release Left click
7. Go back to Game view, observe button reacting as if you're still holding the left click

Expected: When you release left mouse button, the button treats it as if you're not holding anything
Actual: Button reacts as if you're still holding left click, even though you're not holding anything

Reproduced on: 2017.2.0a3, 2017.1.0b9, 5.6.1f1, 5.5.3p4, 5.4.5p1