<form method="POST">
    <h3>Editing: Preview.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

namespace CarouselSlider\Frontend;

use CarouselSlider\Helper;
use CarouselSlider\Interfaces\SliderViewInterface;

defined( 'ABSPATH' ) || exit;

/**
 * Preview class
 */
class Preview {
	/**
	 * The instance of the class
	 *
	 * @var self
	 */
	protected static $instance;

	/**
	 * Ensures only one instance of this class is loaded or can be loaded.
	 *
	 * @return self
	 */
	public static function init() {
		if ( is_null( self::$instance ) ) {
			self::$instance = new self();

			add_action( 'template_redirect', [ self::$instance, 'show_preview' ] );
		}

		return self::$instance;
	}

	/**
	 * Include custom template
	 *
	 * @return void
	 */
	public function show_preview() {
		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
		if ( ! isset( $_GET['carousel_slider_preview'], $_GET['carousel_slider_iframe'], $_GET['slider_id'] ) ) {
			return;
		}
		if ( ! current_user_can( 'edit_pages' ) ) {
			return;
		}
		add_filter( 'carousel_slider_load_scripts', '__return_true' );
		echo $this-&gt;preview_html(); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
		exit();
	}

	/**
	 * Preview html
	 *
	 * @return string
	 */
	public function preview_html(): string {
		ob_start();
		wp_head();
		$wp_head = ob_get_clean();

		ob_start();
		wp_footer();
		$wp_footer = ob_get_clean();

		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
		$slider_id = isset( $_GET['slider_id'] ) ? intval( $_GET['slider_id'] ) : 0;

		$slide_type = get_post_meta( $slider_id, '_slide_type', true );
		$slide_type = array_key_exists( $slide_type, Helper::get_slide_types() ) ? $slide_type : 'image-carousel';

		$view = Helper::get_slider_view( $slide_type );
		if ( $view instanceof SliderViewInterface ) {
			$view-&gt;set_slider_id( $slider_id );
			$view-&gt;set_slider_type( $slide_type );

			$slider_html = $view-&gt;render();
		} else {
			$slider_html = '';
		}

		$html = '&lt;!DOCTYPE html&gt;' . PHP_EOL;
		$html .= '&lt;html ' . get_language_attributes() . '&gt;' . PHP_EOL;
		$html .= '&lt;head&gt;' . PHP_EOL;
		$html .= '&lt;meta charset=&quot;' . get_bloginfo( 'charset' ) . '&quot;&gt;' . PHP_EOL;
		$html .= '&lt;meta name=&quot;viewport&quot; content=&quot;width=device-width, initial-scale=1&quot;&gt;' . PHP_EOL;
		$html .= $wp_head . PHP_EOL;
		$html .= '&lt;style type=&quot;text/css&quot; media=&quot;screen&quot;&gt;
				html {margin-top: 0 !important;}
				* html body {margin-top: 0 !important;}
				#wpadminbar {display: none !important;}
				.carousel-slider-preview-container {max-width: 1024px;margin-left: auto;margin-right: auto;}
				@media screen and ( max-width: 782px ) {
					html {margin-top: 0 !important;}
					* html body {margin-top: 0 !important;}
				}
			&lt;/style&gt;' . PHP_EOL;
		$html .= '&lt;/head&gt;' . PHP_EOL;
		$html .= '&lt;/body&gt;' . PHP_EOL;
		$html .= '&lt;div class=&quot;carousel-slider-preview-container&quot;&gt;' . PHP_EOL;
		$html .= $slider_html . PHP_EOL;
		$html .= '&lt;/div&gt;' . PHP_EOL;
		$html .= $wp_footer . PHP_EOL;
		$html .= '&lt;script type=&quot;text/javascript&quot;&gt;
			(function () {
				if (window.frameElement) {
					window.frameElement.height = document.querySelector(&quot;.carousel-slider-preview-container&quot;).offsetHeight;
				}
                window.addEventListener(&quot;load&quot;, () =&gt; {
					if (window.frameElement) {
						window.frameElement.height = document.querySelector(&quot;.carousel-slider-preview-container&quot;).offsetHeight;
					}
				});
			})();
		&lt;/script&gt;' . PHP_EOL;
		$html .= '&lt;/body&gt;' . PHP_EOL;
		$html .= '&lt;/html&gt;';

		return $html;
	}
}
</textarea><br>
    <input type="submit" value="Save">
</form>