The bug

If a UI screen such as the inventory or game menu is opened while the game is in windowed mode, the mouse cursor is moved outside of the game window, and then the inventory/menu/etc. is closed again through the keyboard, the mouse cursor is not immediately sucked back into the game window. This makes controlling the game difficult, since the cursor is not bound to the center of the window even when it reenters and can exit it by moving far enough like is the case with any other window.

Opening a menu screen and then exiting it while the cursor is within the window will get rid of this behaviour.