import QtQuick
import QtQuick.Window
import QtQuick.Controls

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

	Button {
		text: "test"

		onClicked: popup.open()

		Popup {
			id: popup

			height: 100
			parent: Overlay.overlay
			width: 100
			x: Math.round((parent.width - width) / 2)
			y: Math.round((parent.height - height) / 2)
		}
	}
}

Warning: Main.qml:20:20: Member "overlay" not found on type "QQuickOverlay" [missing-property]
			parent: Overlay.overlay
                			^^^^^^^