Flickering when dragging a sub window

// code placeholder
import QtQuick import QtQuick.Controls
Window {
id: mainWindow
width: 640
height: 480
visible: true
 
Window {
    id: childWindow
    color: "steelblue"
    parent: mainWindow
    visible: true
    flags: Qt.FramelessWindowHint

    MouseArea {
        anchors.fill: parent
        property var clickPos: Qt.point(0, 0)

        onPressed:(mouse) => {
                      clickPos  = Qt.point(mouse.x,mouse.y)
                  }

        onPositionChanged: (mouse) =>{
                               var delta = Qt.point(mouse.x-clickPos.x, mouse.y-clickPos.y)
                               childWindow.x += delta.x;
                               childWindow.y += delta.y;
                           }
    }

    Column {
        anchors.centerIn: parent
        Label {
            anchors.horizontalCenter: parent.horizontalCenter
            text: `Drag to move !\n${childWindow.x}, ${childWindow.y}`
        }

        Button {
            text: childWindow.parent === null ? "Attach" : "Detach"
            onClicked: {
                childWindow.parent = childWindow.parent === mainWindow ? null : mainWindow
            }
        }
    }
}

}