The 'onMouseDown' callback

Event number: 3
Generated for: Any visible, enabled object (except as described below)

When the user presses the mouse button in the enclosing rectangle of an object, the onMouseDown callback (if any) is invoked (note that for standard controls such as Buttons and Lists, you should normally rely on the Click event rather than the Mouse Down event to detect user input). It can be used to make non-standard controls responsive to mouse input (for example, a Rectangle or Static text). The event-specific parameters in ⎕EV are (in index origin 1):

        ⎕EV[6]        Mouse vertical position in window coordinates
        ⎕EV[7]        Mouse horizontal position in window coordinates
        ⎕EV[8]        Button number (always 1 on the Macintosh)
        ⎕EV[9]        Modifiers (as for Key Down)

Note: Under MacOS, this event is not generated for certain controls, including Button, Radio and Check controls.