property boxX

This property returns the mouse cursor's horizontal offset from the event's targetBox's top-left corner.

<box on:mouseDown="onMouseDown();"/>

function onMouseDown()
{
    var eventObj = shell.currentEvent();
    if (eventObj)
    {
        var boxX = eventObj.boxX;
        if (boxX >= 0)
        {
            // the mouse is at the right side of (or on) the left edge of the targetBox
        }
        else
        {
            // the mouse is at the left side of the left edge of the targetBox
        }
    }
}