Live blog with polling auto-refresh

<?php
/**
 * Live blog feed endpoint.
 *
 * @package Acme_Theme
 */

defined( 'ABSPATH' ) || exit;

function acme_live_blog_register_route(): void {
	register_rest_route(
		'acme/v1',
		'/live/(?P<post>\d+)',
		array(
			'methods'             => WP_REST_Server::READABLE,
			'permission_callback' => '__return_true', // Public, read-only.
			'args'                => array(
				'post'  => array( 'validate_callback' => 'rest_validate_request_arg', 'sanitize_callback' => 'absint' ),
				'since' => array( 'default' => 0, 'sanitize_callback' => 'absint' ),
			),
			'callback'            => 'acme_live_blog_entries',
		)
	);
}
add_action( 'rest_api_init', 'acme_live_blog_register_route' );

function acme_live_blog_entries( WP_REST_Request $request ): WP_REST_Response {
	$post_id = (int) $request['post'];
	$since   = (int) $request['since'];

	if ( 'publish' !== get_post_status( $post_id ) ) {
		return new WP_REST_Response( array( 'entries' => array(), 'now' => time() ), 404 );
	}

	$comments = get_comments(
		array(
			'post_id'    => $post_id,
			'status'     => 'approve',
			'type'       => 'live_update',
			'date_query' => array( array( 'after' => gmdate( 'Y-m-d H:i:s', $since ), 'inclusive' => false ) ),
			'orderby'    => 'comment_date_gmt',
			'order'      => 'DESC',
			'number'     => 20,
		)
	);

	$entries = array_map(
		static fn( WP_Comment $c ): array => array(
			'id'   => (int) $c->comment_ID,
			'time' => mysql2date( get_option( 'time_format' ), $c->comment_date ),
			'html' => wp_kses_post( wpautop( $c->comment_content ) ),
		),
		$comments
	);

	$response = new WP_REST_Response( array( 'entries' => $entries, 'now' => time() ) );
	$response->header( 'Cache-Control', 'public, max-age=10' );
	return $response;
}

(() => {
	const feed = document.querySelector('[data-live-feed]');
	if (!feed) return;

	const url = feed.dataset.liveFeed;       // e.g. /wp-json/acme/v1/live/42
	const every = 15000;                    // ms between checks
	let since = Number(feed.dataset.now) || Math.floor(Date.now() / 1000);
	let timer = null;

	async function poll() {
		try {
			const res = await fetch(`${url}?since=${since}`, { headers: { Accept: 'application/json' } });
			if (!res.ok) return;
			const data = await res.json();
			since = data.now;
			data.entries.reverse().forEach((entry) => {
				const item = document.createElement('article');
				item.className = 'acme-live__entry';
				item.innerHTML = `<time>${entry.time}</time>${entry.html}`; // html is sanitised server side
				feed.prepend(item);
			});
		} catch (e) { /* try again next tick */ }
	}

	function start() { if (!timer) timer = setInterval(poll, every); }
	function stop() { clearInterval(timer); timer = null; }

	document.addEventListener('visibilitychange', () => (document.hidden ? stop() : (poll(), start())));
	start();
})();