Whenever I open a world with Quick Play, my cursor will enter this tiny box on my screen, the exact size of the MC window, even though the window is not selected. It will act as normal, being able to click and stuff, but it is confined to this box. The only way to free it, from what I've found, is to use Alt+Tab to open the MC window, where, the cursor will snap to the middle of the screen (because it's supposed to). Upon going to the game menu everything works as normal.