Overlay's opacity is ignored

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.15

Window {
    id: window
    width: 640
    height: 480
    visible: true

    Text {text: "Hello"}

    Popup {
        id: popup
        anchors.centerIn: parent
        width: 320
        height: 240
        modal: false
        dim: true
        closePolicy: Popup.NoAutoClose

        Text {
            anchors.centerIn: parent
            text: "World"
        }

        Overlay.modeless: Rectangle {
            color: "blue"
            opacity: 0.5
            
            // color: "#770000ff" // WORKAROUND
        }
    }
    Component.onCompleted: popup.open()
}