Flow layout not responding to change of flow property

import Qt 4.7

Rectangle {
    width: 640; height: 480;

    property bool isInPortrait: false

    Flow {
        flow: isInPortrait ? Flow.LeftToRight : Flow.TopToBottom
        spacing: 20
        anchors.horizontalCenter: parent.horizontalCenter
        Rectangle { color: "red"; width: 200; height: 50 }
        Rectangle { color: "blue"; width: 200; height: 50 }
    }

    MouseArea { anchors.fill: parent; onClicked: { print("clicked"); isInPortrait = !isInPortrait } }
}