To exit the game:
Click the cross (✖) in the top-right corner of the window.

To minimize the game:
Click the dash (–) in the top-right corner. The game will be minimized to the taskbar.

To enter fullscreen mode:
Click the two arrows pointing away from each other (⤢). The window will expand to fill the screen.

To exit fullscreen mode:
Move your mouse cursor to the top-right corner of the screen to reveal the window controls.
Then click the two diagonal arrows pointing toward each other (↗↙) to return to window mode.