Layout gives unexpected preview result

Rectangle {
    id: rectangle
    width: 500
    height: 500
    RowLayout {
        anchors.right: parent.right
        anchors.left: parent.left
        Button {
            Layout.alignment: Qt.AlignLeft
            text: "left"
        }
        Button {
            Layout.alignment: Qt.AlignRight
            text: "right"
        }
    }
}