Cannot use QtObject as containmentMask

import QtQml
import QtQuick

Window {
    id: window
    visible: true
    width: 640
    height: 480

    QtObject {
        id: mask
        function contains(point: point) {
            print("hello")
            return point.x >= (control.width - width) / 2
                && point.x < (control.width + width) / 2
                && point.y >= control.y
                && point.y < control.y + control.height
        }
    }

    Rectangle {
        id: control
        width: 10
        height: 100
        color: tapHandler.pressed ? Qt.darker("#444") : (hoverHandler.hovered ? Qt.lighter("#444") : "#444")
        anchors.centerIn: parent

        TapHandler {
            id: tapHandler
        }
        HoverHandler {
            id: hoverHandler
        }

        containmentMask: mask
    }
}