fruneen

const el = document.querySelector('.element');
const wrapper = document.querySelector('.wrapper');


window.addEventListener('scroll', () => {
  const wrapperBox = wrapper.getBoundingClientRect();
  const elBox = el.getBoundingClientRect();

  const isInside = elBox.top > wrapperBox.top && elBox.bottom < wrapperBox.bottom;

  el.style.opacity = isInside ? 1 : 0;
});

transition: opacity 0.3s;
opacity: 0;

.буква {
  pointer-events: none;
}