如何实现半屏滚动锁定效果？

<div class="fixed-section">
  <!-- 固定不动的内容 -->
</div>

<div class="scrollable-section">
  <!-- 可滚动的内容 -->
</div>

.fixed-section {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 50%;
  background-color: #fff;
  z-index: 999;
}

.scrollable-section {
  height: 100%;
  overflow-y: scroll;
}

window.addEventListener('scroll', function() {
  var scrollPosition = window.scrollY;
  var fixedSection = document.querySelector('.fixed-section');

  if (scrollPosition > 0) {
    fixedSection.style.display = 'none';
  } else {
    fixedSection.style.display = 'block';
  }
});