Tumbler's displacement is incorrect when model has 2 items

import QtQuick 2.5
import QtQuick.Controls 2.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    Frame {
        anchors.centerIn: parent

        Tumbler {
            id: tumbler

            implicitWidth: 100
            model: [0,1]
            currentIndex: 1

            delegate: Text {
                text: index + " (" + Tumbler.displacement.toFixed(2) + ")"
                horizontalAlignment: Text.AlignHCenter
                verticalAlignment: Text.AlignVCenter
            }
        }
    }
}