The following example uses the attached \l {Overlay::overlay}{Overlay.overlay}
property to position a popup in the center of the window, despite the position
of the button that opens the popup:

\code
Button {
    onClicked: popup.open()

    Popup {
        id: popup

        parent: Overlay.overlay

        x: (parent.width - width) / 2
        y: (parent.height - height) / 2
        width: 100
        height: 100
    }
}
\endcode