Pressing Alt+Escape Opens up the pause screen, but leaves the mouse cursor frozen until an either left or right-click is pressed on the mouse.

Steps to Reproduce:

1. Open up a world or join a server
2. Hold Alt
3. Press the Escape key
4. let go of both simultaneously
5. Move your mouse (cursor won't move)
6. Click either the left or right mouse button
7. The mouse cursor is now moveable