def METHOD_NAME(self, event):
    """
    The Qt event handler when the mouse is pressed down
    If a slicing plane was clicked select it and save the mouse position.
    Otherwise pass the event to the parent class.
    :param QMouseEvent event: the qt mouse event
    """
    x, y = event.x(), event.y()
    hits = self.itemsAt((x - 10, y - 10, 20, 20))
    if hits:
        self._mouse_pos = x, y
        self._slice_planes.select(hits[0])
    return GLViewWidget.METHOD_NAME(self, event)