Ok, one thing I found was that double clicks don't count as a click. Maybe there's a setting I'm missing, but I created a button, then I wanted a sprite to rotate every time it was pressed down, regardless of if the click was a double click or not. The only action for the button which worked as expected was "on up". I realize there's a seperate action for double clicks, but the button should respond equally for the action "on down" as "on up"
Edit: I experimented further and this only happens if the mouse is pressed at the exact same position as before on the second click. For example, if you change position every time you click, it doesn't care about whether it's a double click or not.
Also, the double click action works strangely, not as expected.