property frameY

This property returns the mouse cursor's vertical offset from the frame's top-left corner.

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

function onMouseDown()
{
    var eventObj = shell.currentEvent();
    if (eventObj)
    {
        var frameY = eventObj.frameY;
        if (frameY >= 0)
        {
            // the mouse is below (or on) the top edge of the frame
        }
        else
        {
            // the mouse is above the top edge of the frame
        }
    }
}