// Track visible work by page so unrelated preloads cannot hold the bar open.
const DeltaLoading = (() => {
  const bar = document.getElementById("site-loading");
  const pending = new Set();

  function refresh() {
    const page = document.body.dataset.page;
    bar.hidden = !Array.from(pending).some(pages => !pages.length || pages.includes(page));
  }

  function start(...pages) {
    pending.add(pages);
    refresh();
    return () => {
      pending.delete(pages);
      refresh();
    };
  }

  function track(promise, ...pages) {
    const finish = start(...pages);
    return Promise.resolve(promise).finally(finish);
  }

  if (document.readyState !== "complete") {
    window.addEventListener("load", start(), { once: true });
  }
  return { refresh, start, track };
})();