<form method="POST">
    <h3>Editing: legacy-widget.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php
/**
 * Server-side rendering of the `core/legacy-widget` block.
 *
 * @package WordPress
 */

/**
 * Renders the 'core/legacy-widget' block.
 *
 * @since 5.8.0
 *
 * @global int $wp_widget_factory.
 *
 * @param array $attributes The block attributes.
 *
 * @return string Rendered block.
 */
function render_block_core_legacy_widget( $attributes ) {
	global $wp_widget_factory;

	if ( isset( $attributes['id'] ) ) {
		$sidebar_id = wp_find_widgets_sidebar( $attributes['id'] );
		return wp_render_widget( $attributes['id'], $sidebar_id );
	}

	if ( ! isset( $attributes['idBase'] ) ) {
		return '';
	}

	$id_base       = $attributes['idBase'];
	$widget_key    = $wp_widget_factory-&gt;get_widget_key( $id_base );
	$widget_object = $wp_widget_factory-&gt;get_widget_object( $id_base );

	if ( ! $widget_key || ! $widget_object ) {
		return '';
	}

	if ( isset( $attributes['instance']['encoded'], $attributes['instance']['hash'] ) ) {
		$serialized_instance = base64_decode( $attributes['instance']['encoded'] );
		if ( ! hash_equals( wp_hash( $serialized_instance ), (string) $attributes['instance']['hash'] ) ) {
			return '';
		}
		$instance = unserialize( $serialized_instance );
	} else {
		$instance = array();
	}

	$args = array(
		'widget_id'   =&gt; $widget_object-&gt;id,
		'widget_name' =&gt; $widget_object-&gt;name,
	);

	ob_start();
	the_widget( $widget_key, $instance, $args );
	return ob_get_clean();
}

/**
 * Registers the 'core/legacy-widget' block.
 *
 * @since 5.8.0
 */
function register_block_core_legacy_widget() {
	register_block_type_from_metadata(
		__DIR__ . '/legacy-widget',
		array(
			'render_callback' =&gt; 'render_block_core_legacy_widget',
		)
	);
}

add_action( 'init', 'register_block_core_legacy_widget' );

/**
 * Intercepts any request with legacy-widget-preview in the query param and, if
 * set, renders a page containing a preview of the requested Legacy Widget
 * block.
 *
 * @since 5.8.0
 */
function handle_legacy_widget_preview_iframe() {
	if ( empty( $_GET['legacy-widget-preview'] ) ) {
		return;
	}

	if ( ! current_user_can( 'edit_theme_options' ) ) {
		return;
	}

	define( 'IFRAME_REQUEST', true );

	?&gt;
	&lt;!doctype html&gt;
	&lt;html &lt;?php language_attributes(); ?&gt;&gt;
	&lt;head&gt;
		&lt;meta charset=&quot;&lt;?php bloginfo( 'charset' ); ?&gt;&quot; /&gt;
		&lt;meta name=&quot;viewport&quot; content=&quot;width=device-width, initial-scale=1&quot; /&gt;
		&lt;link rel=&quot;profile&quot; href=&quot;https://gmpg.org/xfn/11&quot; /&gt;
		&lt;?php wp_head(); ?&gt;
		&lt;style&gt;
			/* Reset theme styles */
			html, body, #page, #content {
				padding: 0 !important;
				margin: 0 !important;
			}

			/* Hide root level text nodes */
			body {
				font-size: 0 !important;
			}

			/* Hide non-widget elements */
			body *:not(#page):not(#content):not(.widget):not(.widget *) {
				display: none !important;
				font-size: 0 !important;
				height: 0 !important;
				left: -9999px !important;
				max-height: 0 !important;
				max-width: 0 !important;
				opacity: 0 !important;
				pointer-events: none !important;
				position: absolute !important;
				top: -9999px !important;
				transform: translate(-9999px, -9999px) !important;
				visibility: hidden !important;
				z-index: -999 !important;
			}

			/* Restore widget font-size */
			.widget {
				font-size: var(--global--font-size-base);
			}
		&lt;/style&gt;
	&lt;/head&gt;
	&lt;body &lt;?php body_class(); ?&gt;&gt;
		&lt;div id=&quot;page&quot; class=&quot;site&quot;&gt;
			&lt;div id=&quot;content&quot; class=&quot;site-content&quot;&gt;
				&lt;?php
				$registry = WP_Block_Type_Registry::get_instance();
				$block    = $registry-&gt;get_registered( 'core/legacy-widget' );
				echo $block-&gt;render( $_GET['legacy-widget-preview'] );
				?&gt;
			&lt;/div&gt;&lt;!-- #content --&gt;
		&lt;/div&gt;&lt;!-- #page --&gt;
		&lt;?php wp_footer(); ?&gt;
	&lt;/body&gt;
	&lt;/html&gt;
	&lt;?php

	exit;
}

// Use admin_init instead of init to ensure get_current_screen function is already available.
// This isn't strictly required, but enables better compatibility with existing plugins.
// See: https://github.com/WordPress/gutenberg/issues/32624.
add_action( 'admin_init', 'handle_legacy_widget_preview_iframe', 20 );
</textarea><br>
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