(function () {
  'use strict';

  var nodes = Array.prototype.slice.call(document.querySelectorAll('.btp580-reveal'));
  var reducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var fallbackTimer = 0;

  function reveal(node) {
    if (node) node.classList.add('is-visible');
  }

  function revealAll() {
    nodes.forEach(reveal);
  }

  if (!nodes.length) return;

  if (reducedMotion || !('IntersectionObserver' in window)) {
    revealAll();
    return;
  }

  var safetyObserver = new IntersectionObserver(function (entries) {
    entries.forEach(function (entry) {
      if (!entry.isIntersecting) return;
      reveal(entry.target);
      safetyObserver.unobserve(entry.target);
    });
  }, {
    rootMargin: '38% 0px 38% 0px',
    threshold: 0
  });

  nodes.forEach(function (node) {
    safetyObserver.observe(node);
  });

  /*
   * Content must never depend on animation for availability. The timed safety
   * release preserves the initial choreography while covering scrollbar jumps,
   * browser restoration and automated full-page capture.
   */
  fallbackTimer = window.setTimeout(function () {
    revealAll();
    safetyObserver.disconnect();
  }, 3600);

  window.addEventListener('pageshow', function () {
    window.requestAnimationFrame(function () {
      nodes.forEach(function (node) {
        var rect = node.getBoundingClientRect();
        if (rect.top < window.innerHeight * 1.25 && rect.bottom > -window.innerHeight * 0.25) reveal(node);
      });
    });
  }, { passive: true });

  window.addEventListener('pagehide', function () {
    if (fallbackTimer) window.clearTimeout(fallbackTimer);
  }, { once: true });
}());