Mastodon: плавающая кнопка "Наверх"

// Add a floating "Back to top" button on the timeline
(function () {
  if (document.getElementById('jz-mst-top')) { return; }
  var btn = document.createElement('button');
  btn.id = 'jz-mst-top';
  btn.textContent = 'Top';
  btn.setAttribute('aria-label', 'Scroll to top');
  btn.style.cssText = 'position:fixed;right:24px;bottom:24px;z-index:9999;background:#6364ff;color:#fff;border:none;padding:10px 14px;border-radius:24px;font-weight:600;box-shadow:0 4px 12px rgba(0,0,0,0.25);cursor:pointer;opacity:0;transition:opacity .2s;';
  document.body.appendChild(btn);
  function scroller() {
    return document.querySelector('.columns-area__panels__main .scrollable') || document.querySelector('.column .scrollable') || document.scrollingElement;
  }
  btn.addEventListener('click', function () {
    var s = scroller();
    if (s) { s.scrollTo({ top: 0, behavior: 'smooth' }); }
  });
  function check() {
    var s = scroller();
    var y = s ? (s.scrollTop || 0) : (window.scrollY || 0);
    btn.style.opacity = y > 600 ? '1' : '0';
  }
  document.addEventListener('scroll', check, true);
  setInterval(check, 800);
})();