FramelessWindowHint window not responding with qml Widget

JonC

Hello,
I have embedded a QML window within a QWidget to display it in a QMainWindow. However, when I set the window flag as FramelessWindowHint and click outside the application on my other screen, and then click inside my Qt application again, the console logs "clicked" but there is no change in the color of my rectangle. This issue only occurs when using the FramelessWindowHint flag.

main.cpp code

int main(int argc, char* argv[])
{
    QApplication app(argc,argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    engine.load(url);

    MainWindow* mainWin = new MainWindow(&engine);
    mainWin->setWindowFlags(Qt::FramelessWindowHint | Qt::Window);
    mainWin->show();
    return app.exec();
}

MainWindow.cpp code

MainWindow::MainWindow(QQmlApplicationEngine* engine, QWidget* parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QScreen *screen = QGuiApplication::primaryScreen();
    QSize screenSize = screen->availableSize();
    this->setFixedSize(screenSize);
    ui->centralwidget->setFixedSize(screenSize);

    // qml window
    QWindow *qmlWindow = qobject_cast<QWindow*>(engine->rootObjects().at(0));
    QWidget* testWidget = QWidget::createWindowContainer(qmlWindow, this);
    testWidget->setContentsMargins(0,0,0,0);
    testWidget->setFixedSize(screenSize);
}

main.qml code

Window
{    
    width: Screen.width
    height: Screen.height

    Rectangle {
        id: id_rect
        width: 200
        height: 300
        anchors.centerIn: parent
        color: "blue"
        property bool isBlue: true

        MouseArea {
            anchors.fill: parent
            onClicked: {
                console.log("clicked");
                id_rect.color = id_rect.isBlue ? "red" : "blue";
                id_rect.isBlue = !id_rect.isBlue;
            }
        }
    }
}

Thanks for your help.