ListView's ScrollBar handle size changes with varying item height

import QtQuick 2.11
import QtQuick.Controls 2.4

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Scroll")

    ListView {
        anchors.fill: parent
        model: 20
        delegate: ItemDelegate {
            width: parent.width
            height: lbl.height

            Label {
                id: lbl
                width: parent.width
                text: {
                    var str = "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.";

                    var res = str;
                    for (var i=0; i<index; i++) {
                        res += str;
                    }
                    return res;
                }
                wrapMode: Text.WordWrap
            }
        }

        ScrollBar.vertical: ScrollBar {}
    }
}