@RainbowRhyme It works when you mark many X's, by using X+arrow keys or mouse dragging. If the option is on, it changes the pixel depending on the pixel you start dragging your mouse. So if you start dragging your mouse from the empty pixel, the dragging will mark any empty pixels on its track with X but won't change pixels already marked X's, and vice versa. Without the option, no matter where you start your dragging, it marks or unmarks X depending on the pixel your mouse cursor is currently on.
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