Rectangle visually not in sync with window background

import QtQuick
import QtQuick.Controls.Basic

Window {
    id: root

    property bool isExpanded: false

    readonly property int normalWidth: 600
    readonly property int normalHeight: 300

    readonly property int expandedWidth: 900
    readonly property int expandedHeight: 600

    visible: true

    x: Screen.width / 2 - normalWidth / 2
    y: Screen.height / 2 - normalHeight / 2

    width: root.normalWidth
    height: root.normalHeight
    color: "magenta"

    Rectangle {
        anchors.fill: parent
        color: "black"
    }

    StateGroup {
        states: State {
            name: "expanded"
            when: root.isExpanded

            PropertyChanges {
                root {
                    width: root.expandedWidth
                    height: root.expandedHeight
                }
            }
        }

        transitions: Transition {
            reversible: true
            to: "expanded"

            NumberAnimation {
                properties: "width, height"
                duration: 5000
                easing.type: Easing.InOutQuad
            }
        }
    }

    Button {
        anchors.centerIn: parent

        text: "Animate Window"

        onClicked: root.isExpanded = !root.isExpanded
    }
}
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