Untitled

function waitForElementVisible(selector, threshold = 0.1) {
  return new Promise(resolve => {
    const el = document.querySelector(selector);
    if (!el) {
      // Se não existir, fica de olho até aparecer no DOM
      const intervalId = setInterval(() => {
        const elRetry = document.querySelector(selector);
        if (elRetry) {
          clearInterval(intervalId);
          observe(elRetry);
        }
      }, 500);
    } else {
      observe(el);
    }

    function observe(element) {
      const observer = new IntersectionObserver((entries, obs) => {
        entries.forEach(entry => {
          if (entry.isIntersecting) {
            obs.disconnect();
            resolve(true);
          }
        });
      }, { threshold });

      observer.observe(element);
    }
  });
}

waitForElementVisible('.gren-product-details-buy-button').then(() => {
  //pronto para aplicar mudanças
  console.log('teste');
});