Launch MC in windowed mode with default window size, load some level. With MC window in focus, in a usual game, press WinKey + Left. The window will fill the left half of the screen and the game will not pause automatically (like in does when focus is lost). Move your mouse to see player's view changing, then press Esc and move mouse pointer out of the MC window to the right half of the screen, do not click anything. Press Esc again, then press WinKey + Up. The player's view will start rotate. This rotation can be stopped by pausing the game (Esc or focus lost) or opening inventory. Do not do that, but press WinKey + Right. The rotation will stop, the focus of the window will be kept, but the mouse movement will be limited by the "hypothetical" client area of the MC window, as if it were on the left.