Collect the block editor assets that need to be loaded into the editor’s iframe.

function _wp_get_iframed_editor_assets() {
	global $pagenow;

	$script_handles = array();
	$style_handles  = array(
		'wp-block-editor',
		'wp-block-library',
		'wp-edit-blocks',
	);

	if ( current_theme_supports( 'wp-block-styles' ) ) {
		$style_handles[] = 'wp-block-library-theme';
	}

	if ( 'widgets.php' === $pagenow || 'customize.php' === $pagenow ) {
		$style_handles[] = 'wp-widgets';
		$style_handles[] = 'wp-edit-widgets';
	}

	$block_registry = WP_Block_Type_Registry::get_instance();

	foreach ( $block_registry->get_all_registered() as $block_type ) {
		$style_handles = array_merge(
			$style_handles,
			$block_type->style_handles,
			$block_type->editor_style_handles
		);

		$script_handles = array_merge(
			$script_handles,
			$block_type->script_handles
		);
	}

	$style_handles = array_unique( $style_handles );
	$done          = wp_styles()->done;

	ob_start();

	// We do not need reset styles for the iframed editor.
	wp_styles()->done = array( 'wp-reset-editor-styles' );
	wp_styles()->do_items( $style_handles );
	wp_styles()->done = $done;

	$styles = ob_get_clean();

	$script_handles = array_unique( $script_handles );
	$done           = wp_scripts()->done;

	ob_start();

	wp_scripts()->done = array();
	wp_scripts()->do_items( $script_handles );
	wp_scripts()->done = $done;

	$scripts = ob_get_clean();

	return array(
		'styles'  => $styles,
		'scripts' => $scripts,
	);
}