I noticed that whenever I pressed arrow keys on the keyboard, my mouse cursor moves in funny ways. This felt quite creepy, as if some software was trying to intercept my keyboard input, and in fact this is exactly what happened. However, the evil hacker software turned out to be Microsoft paint. According to this helpful answer “it has a feature that allows you to move the curser with the arrow keys and it sometimes doesn’t realize when it looses focus’. Closing MS Paint solved the problem. Phew!