Popup with margins opened at wrong position

import QtQuick 2.7
import QtQuick.Controls 2.0

ApplicationWindow {
    visible: true
    width: 400
    height: 300

    Button {
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.bottom: parent.bottom
        text: "Open popup"
        onClicked: popup.open()

        Popup {
            id: popup

            property point position: parent.mapFromItem(null,
                                                        (ApplicationWindow.window.width - width) / 2,
                                                        (ApplicationWindow.window.height - height) / 2)

            x: position.x
            y: position.y
            margins: 20

            contentItem: Item {
                implicitWidth: 100
                implicitHeight: 100
                Rectangle {
                    anchors.fill: parent
                    color: "skyblue"
                }
            }
        }
    }
}