Popup: Some properties can not be bound

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12

Window {
    width: 320
    height: 240
    visible: true
    title: "Unbindable Properties"

    Dialog {
        id: dialog
        anchors.centerIn: parent
        visible: true
        closePolicy: Popup.NoAutoClose
        contentItem: Column {
            spacing: dialog.spacing
            Button {
                text: "Increase Spacing: " + dialog.spacing
                onClicked: ++dialog.spacing // not working
            }
            Button {
                text: "Increase Padding: " + dialog.padding
                onClicked: ++dialog.padding // works
            }
        }
    }
}