ListView's contentY isn't updated when the delegateHeight starts off at 0

import QtQuick 2.0
import QtQuick.Window 2.0

Window {
    id: window
    width: 800
    height: 400
    visible: true
    title: "currentIndex: " + listView.currentIndex
     + " contentY: " + listView.contentY

    property real delegateHeight: 0

    ListView {
        id: listView
        width: 800
        height: 400
//        width: parent.width
//        height: parent.height
        currentIndex: 50
        model: 100
        delegate: Text {
            id: delegateText
            text: modelData
            width: ListView.view.width
            height: delegateHeight
            font.bold: ListView.isCurrentItem
            horizontalAlignment: Text.AlignHCenter
            verticalAlignment: Text.AlignVCenter
        }
    }

    Timer {
        running: true
        interval: 0
        onTriggered: delegateHeight = 133
    }
}