ComboBox hide behind Dialog if Dialog z > 0

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Button {
        text: "show dialog"
        onClicked: dialog.visible = true
    }

    Dialog {
        id: dialog
        title: "Title"
        standardButtons: Dialog.Ok | Dialog.Cancel
        z: 1
        ComboBox {
            id: comboBox
            model: ["First", "Second", "Third"]
        }
    }
}