QtQuick transparent toplevel window broken in dev

import QtQuick 2.7
import QtQuick.Window 2.0
import QtQuick.Controls 1.1

ApplicationWindow {
    id: backlight
    flags: Qt.FramelessWindowHint
    visible: true
    title: qsTr("transparent")
    width: 500
    height: 50
    x: (Screen.width - width) / 2
    y: (Screen.height - height) / 2
    color: "transparent"

    Rectangle {
        anchors.centerIn: parent
        width: parent.width
        height: 50
        color: "transparent"

        Rectangle {
            anchors.fill: parent
            radius: 25
            opacity: 0.3
            color: "gray"
        }
    }
}