Content Y jumps constantly if first delegate height == 0 and other delegates have height > 0

import QtQuick 2.0

ListView {
    property int delegateHeight: 20
    width: 240; height: 320
    model: ListModel {
        Component.onCompleted: {
            for (var i = 0; i < 10; i++)
                append({})
        }
    }
    delegate: Rectangle {
        width: 240; height: index == 0 ? delegateHeight : 20; color: "red"
        Text { text: index }
    }

    focus: true
    Keys.onReturnPressed: delegateHeight = 0

    Text { anchors.bottom: parent.bottom; text: parent.contentY }
}