export function backToTop(){
  const btn = document.getElementById('btn--back-to-top');
  if (!btn) return;

  let lastY = window.scrollY;
  let peakY = window.scrollY;

  window.addEventListener('scroll', () => {
    const currentY = window.scrollY;

    if (currentY > lastY) {
      peakY = currentY;
      btn.classList.remove('is-visible');
    } else {
      if (currentY <= 800) {
        btn.classList.remove('is-visible');
      } else if (peakY - currentY >= 200 && peakY > 600) {
        btn.classList.add('is-visible');
      }
    }

    lastY = currentY;
  }, { passive: true });
}