buildPageTopAndBottom example

// Now render the rendered page.html.twig template inside the html.html.twig     // template, and use the bubbled #attached metadata from $page to ensure we     // load all attached assets.     $html = [
      '#type' => 'html',
      'page' => $page,
    ];

    // The special page regions will appear directly in html.html.twig, not in     // page.html.twig, hence add them here, just before rendering html.html.twig.     $this->buildPageTopAndBottom($html);

    // Render, but don't replace placeholders yet, because that happens later in     // the render pipeline. To not replace placeholders yet, we use     // RendererInterface::render() instead of RendererInterface::renderRoot().     // @see \Drupal\Core\Render\HtmlResponseAttachmentsProcessor.     $render_context = new RenderContext();
    $this->renderer->executeInRenderContext($render_context, function D) use (&$html) {
      // RendererInterface::render() renders the $html render array and updates       // it in place. We don't care about the return value (which is just       // $html['#markup']), but about the resulting render array.       // @todo Simplify this when https://www.drupal.org/node/2495001 lands.