Z-ordering is broken

import QtQuick 2.1
import QtQuick.Window 2.1

Window {
    width: 640
    height: 400

    Repeater {
        id: repeater
        anchors.fill: parent
        model: ["chocolate", "steelblue", "rosybrown", "seagreen", "mediumslateblue"]

        MouseArea {
            id: mouseArea

            z: repeater.count - index
            drag.target: mouseArea
            acceptedButtons: Qt.LeftButton | Qt.RightButton
            onDoubleClicked: mouse.button == Qt.LeftButton ? ++z : --z

            x: index * width / repeater.count
            y: index * height / repeater.count
            width: parent.width / repeater.count * 2
            height: parent.height / repeater.count * 2

            Rectangle {
                anchors.fill: parent
                opacity: 0.8
                border.width: 1
                color: modelData

                Text {
                    text: mouseArea.z
                    anchors.margins: 10
                    anchors.right: parent.right
                    anchors.bottom: parent.bottom
                }
            }
        }
    }
}