scroll.js

// if the URL contains a hash, scroll to the associated heading
if (location.hash) {
  let media = body.querySelectorAll('img, video');
  let elem = body.querySelector(location.hash);
  let mediaPromise = [];

  for (let m of media) {
    // “<img>/<video> rendered” promises are needed for a scrollIntoView() re-run
    mediaPromise.push(
      new Promise((res) => {
        if (m.tagName === 'IMG') {
          if (m.complete && m.naturalWidth) res();
          m.onload = () => res();
          m.onerror = () => res();
        } else if (m.tagName === 'VIDEO') {
          if (m.readyState >= 3) res();
          m.onloadeddata = () => res();
          m.onerror = () => res();
        }
      })
    );
  }

  setTimeout(() => {
    elem.scrollIntoView({ top: 'start' });
    Promise.all(mediaPromise).then(() =>
      elem.scrollIntoView({ top: 'start' })
    );
  }, 0);
}