OpenGL based QQuickItem can't rendering with other QML elements at the same time

0x5FB5

I'm new in Qt/OpenGL stuff and I've created an OpenGL based QQuickItem class that should render 3D.
If I use it without any QML elements in scene it works, but when I set together something like Rectangle, I only can see black rectangle instead of 3D render. What's wrong?

For example, this is how render works:

GLWindow::GLWindow(QQuickItem *parent) : QQuickItem(parent)
{
    setFlag(QQuickItem::ItemHasContents, true);

    frameTimer = new QTimer(this);

    connect(this, &GLWindow::windowChanged, this, &GLWindow::handleWindowChanged);
    connect(frameTimer, &QTimer::timeout, this, &GLWindow::updateScene);

    frameTimer->start(FRAMERATE_MS);
}

void GLWindow::updateScene()
{
    if (window())
        window()->update();
}

void GLWindow::render()
{
    initializeOpenGLFunctions();

    glClearColor(0.f, 0.f, 0.f, 1.f);

    QPoint cursorPos = QCursor::pos();

    // VIEWPORT
    qreal ratio = window()->devicePixelRatio();

    GLint x = static_cast<GLint>(ratio * parentItem()->x());
    GLint y = static_cast<GLint>(ratio * parentItem()->y());
    GLsizei w = static_cast<GLsizei>(ratio * parentItem()->width());
    GLsizei h = static_cast<GLsizei>(ratio * parentItem()->height());

    glViewport(x, y, w, h);

    glEnable(GL_DEPTH_TEST);
    glEnable(GL_SCISSOR_TEST);

    glScissor(x, y, w, h);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glDisable(GL_SCISSOR_TEST);
    glDisable(GL_DEPTH_TEST);

   // Rendering stuff...
}

void GLWindow::handleWindowChanged(QQuickWindow *win)
{
    if (win) {
        connect(win, &QQuickWindow::afterRendering, this, &GLWindow::render, Qt::DirectConnection);
        win->setClearBeforeRendering(true);
    }
}