// Track visible work by page so unrelated preloads cannot hold the bar open.
const DeltaLoading = (() => {
  const bar = document.getElementById("site-loading");
  const slow = document.getElementById("site-slow");
  const SLOW_MS = 4000;
  const pending = new Set();
  let slowTimer = null;

  // The Space can take tens of seconds on a first visit, so say so when the bar stays up.
  function refresh() {
    const page = document.body.dataset.page;
    bar.hidden = !Array.from(pending).some(pages => !pages.length || pages.includes(page));
    if (bar.hidden) {
      clearTimeout(slowTimer);
      slowTimer = null;
      slow.hidden = true;
    } else if (slowTimer === null && slow.hidden) {
      slowTimer = setTimeout(() => {
        slowTimer = null;
        slow.hidden = bar.hidden;
      }, SLOW_MS);
    }
  }

  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 };
})();