Fullscreen mode does not support alternate aspect ratios; windowed mode allows mouse to escape game window

When using an ultrawide monitor, the game does not allow selecting a 16:9 resolution (e.g., 2560×1440) while in fullscreen mode. The only way to play at 16:9 is to use windowed mode. However, in windowed mode the mouse cursor is not locked to the game window, allowing clicks outside the game which causes the game to lose focus. It would be nice to have a fix to either or both of these issues.