embedded window invisible with transparent window

Create a round rectangle window as shown below:

Window {
    flags: Qt.FramelessWindowHint
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
    color: "transparent"
    Rectangle

{         anchors.fill: parent         radius: 20         color: "blue"     }

}

When embedding an external window into this QML Window, the embedded window is not visible. However, changing the color property of the Window from "transparent" to a non-transparent color makes the embedded window visible.

 

color: "transparent"

color: "red"