(function () {
  var scrollToTopButton = document.getElementById("scrollToTopButton");
  if (!scrollToTopButton) return;

  var ticking = false;
  var lastVisible = null;

  function updateVisibility() {
    ticking = false;
    // 用 visualViewport / 近似阈值，避免反复读 layout 几何属性触发强制重排
    var threshold = Math.max(window.innerHeight || 600, 600);
    var shouldShow = (window.scrollY || window.pageYOffset || 0) > threshold;
    if (shouldShow === lastVisible) return;
    lastVisible = shouldShow;
    scrollToTopButton.classList.toggle("is-visible", shouldShow);
  }

  function onScroll() {
    if (ticking) return;
    ticking = true;
    window.requestAnimationFrame(updateVisibility);
  }

  scrollToTopButton.addEventListener("click", function () {
    window.scrollTo({ top: 0, behavior: "smooth" });
  });

  window.addEventListener("scroll", onScroll, { passive: true });
  window.requestAnimationFrame(updateVisibility);
})();