TOC before first heading

/**

 * TOC After First Heading

 */

function cwp_toc_after_first_heading( $output, $block ) {

	if ( 'core/heading' !== $block['blockName'] ) {

		return $output;

	}

	global $wp_query;

	if ( ! $wp_query->in_the_loop ) {

		return $output;

	}

	// Only run once.

	remove_filter( 'render_block', 'cwp_toc_after_first_heading', 10, 2 );

	$toc = render_block( [ 'blockName' => 'cwp/toc', 'attrs' => [ 'name' => 'cwp/toc', 'mode' => 'preview' ] ] );

	return $toc . $output;

}

add_filter( 'render_block', 'cwp_toc_after_first_heading', 10, 2 );