time-based size animation not working

// Main.qml
import QtQuick
import QtQuick.Controls
import QtQuick.Layouts

ApplicationWindow {
    id: mainWindow

    visible: true
    width: 800
    height: 480

    ColumnLayout {
        anchors.fill: parent
        spacing: 0

        CollapsibleBar {
            id: collapsibleBar
            Layout.alignment: Qt.AlignVCenter | Qt.AlignTop
            contents: Rectangle {
                implicitHeight: 300 // this is causing the problem.
                width: mainWindow.width
                color: 'lightgray'
                Text {
                    anchors.centerIn: parent
                    text: 'this light gray bar should appear below the light blue bar.'
                }
            }
        }
        Rectangle {
            y: collapsibleBar.height
            Layout.preferredHeight: 100
            Layout.fillWidth: true
            color: 'lightgreen'
            Text {
                anchors.centerIn: parent
                text: "this light green bar should appear below everything else."
            }
        }
    }
}

// CollapsibleBar.qml
import QtQuick
import QtQuick.Controls
import QtQuick.Layouts

Rectangle {
    id: bar
    color: "lightblue"

    default property alias contents: contentItem.children
    property bool collapsed: true

    height: contentsArea.height
    width: mainWindow.width

    Column {
        id: contentsArea

        RowLayout {
            id: barHeader

            width: mainWindow.width
            height: 32

            RoundButton {
                implicitWidth: 24
                implicitHeight: 24

                onClicked: {
                    bar.collapsed = !bar.collapsed;
                }
            }

            Text {
//                Layout.Alignment: Qt.AlignHCenter // BTW: this line doesn't compile.
                text: "this light blue bar should appear above everything."
            }
        }

        Column {
            id: contentItem
            clip: true
            height: bar.collapsed ? 0 : implicitHeight

            Behavior on height {
                NumberAnimation {
                    duration: 500
                }
            }
        }
    }