SwipeDelegate::position value jumps abruptly

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Controls.Material 2.0

ApplicationWindow {
    width: 300
    height: 400
    visible: true

    ListView {
        model: 10
        anchors.fill: parent

        delegate: SwipeDelegate {
            id: delegate
            text: index
            width: parent.width

            swipe.left: Rectangle {
                width: parent.width
                height: parent.height
                color: Material.color(Material.Green, SwipeDelegate.pressed ? Material.Shade700 : Material.Shade500)

                Label {
                    text: "Left"
                    color: "white"
                    anchors.margins: 10
                    anchors.left: parent.left
                    anchors.verticalCenter: parent.verticalCenter
                    scale: delegate.swipe.position // <==
                }

                SwipeDelegate.onClicked: delegate.swipe.close()
            }

            swipe.right: Rectangle {
                width: parent.width
                height: parent.height
                anchors.right: parent.right
                color: Material.color(Material.Red, SwipeDelegate.pressed ? Material.Shade700 : Material.Shade500)

                Label {
                    text: "Right"
                    color: "white"
                    anchors.margins: 10
                    anchors.right: parent.right
                    anchors.verticalCenter: parent.verticalCenter
                    scale: -delegate.swipe.position // <==
                }

                SwipeDelegate.onClicked: delegate.swipe.close()
            }
        }
    }
}