Qml ListView increases/decreases contentY when delegates in List have different height and executing positionViewAtEnd()

import QtQuick 1.1

Rectangle
{
    width: 360
    height: 360

    Text
    {
        id: contenty;
        objectName: "contenty";


        text: lv.contentY;
        anchors.top: parent.top

    }

    ListView
    {
        id: lv
        anchors.fill: parent
        anchors.bottomMargin: 30;
        anchors.topMargin: 30;
        spacing: 5
        clip: true;

        model: msgmodel
        delegate:
        Rectangle
        {
            id: rect;
            width: lv.width;
            height: txt.height;
            Text
            {
                id: txt;

                width: 150;
                anchors.top: parent.top;
                anchors.topMargin: 5;

                //wrapMode: Text.WrapAtWordBoundaryOrAnywhere;
                text: lv.contentHeight +  " " + msgtext;
            }
        }
    }

    Text
    {
        id: btn;
        objectName: "btn";

        signal add();

        text: "Click to move at end"
        anchors.bottom: parent.bottom
        MouseArea
        {
            anchors.fill: parent
            onReleased:  lv.positionViewAtEnd();
        }
    }
}