The documentation (linked below) states that “button is a vector of integers indicating which mouse buttons you pressed (1 for left, 2 for middle, 3 for right), or ASCII numbers indicating which keys on the keyboard you pressed”. Therefore the third output should only be 1 when you use the left click to select the point.
The third output should be empty only when you hit "Return" before clicking on any points as stated in the documentation because no points were chosen.
