property originalTargetBox

This property returns target box assigned to the event when creating the event object.

<gadget id="myBox" language="jscript" code="...">
    <attributes blockEvents="true"/>
    <parts>
        <box:hbox s:fill="red" s:height="100" s:width="100"/>
    </parts>
    <behavior>
        <reaction event="mouseDown" action="gadget:onMouseDown();"/>
    </behavior>
</gadget>

// in the gadget script
function onMouseDown()
{
    var eventObj = shell.currentEvent;
    if (eventObj)
    {
        var originalTargetBox = eventObj.originalTargetBox;
        var targetBox = eventObj.targetBox;
        if (originalTargetBox && targetBox && originalTargetBox != targetBox)
        {
            // if you click on the hbox part, the targetBox of the MouseEvent (which is inherited from DOMEvent)
            // is myBox because it sets blockEvents="true", but the originalTargetBox is the hbox
        }
    }
}