<header>
  <h1>Header</h1>
</header>
<main>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>
  <p>scroll</p>

</main>

html,
body {
  margin: 0;
  padding: 0;
}
h1 {
  margin: 0;
  font-size: inherit;
  padding: 1rem 0;
}

header {
  position: sticky;
  text-align: center;
  padding: 2rem 0;
  top: -2rem;
  background: blue;
  font-size: 2rem;
  color: #fff;
  animation: scroll 1s linear forwards;
  animation-timeline: view();
  animation-range-start: 100vh;
  animation-range-end: calc(100vh + 135px);
}
@keyframes scroll {
  to {
    padding: 0.5rem;
    background: red;
    font-size: 1rem;
    top: 0;
  }
}