【JS&CSS】途中からフワッと表れて追従する固定ヘッダーの作り方

<header id="main-header">
  <div class="logo">仮ロゴ</div>
  <!-- PC用表示 -->
  <nav class="pc-menu">
    <ul>
      <li><a href="#">Menu 1</a></li>
      <li><a href="#">Menu 2</a></li>
      <li><a href="#">Menu 3</a></li>
    </ul>
  </nav>
  <!-- モバイル用表示 -->
  <div class="mobile-menu">
    <div class="line"></div>
    <div class="line"></div>
    <div class="line"></div>
  </div>
</header>

<header id="fixed-header">
  <div class="logo">仮ロゴ</div>
  <!-- PC用表示 -->
  <nav class="pc-menu">
    <ul>
      <li><a href="#">Menu 1</a></li>
      <li><a href="#">Menu 2</a></li>
      <li><a href="#">Menu 3</a></li>
    </ul>
  </nav>
  <!-- モバイル用表示 -->
  <div class="mobile-menu">
    <div class="line"></div>
    <div class="line"></div>
    <div class="line"></div>
  </div>
</header>
<main>
  <!-- ページの内容 -->
</main>

header {
  display: flex;
  justify-content: space-between;
  height: 80px;
}

#main-header {
  position: relative;
  background: #333;
  color: #fff;
  line-height: 80px;
}

#fixed-header {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  background: #333;
  color: #fff;
  line-height: 80px;
  transition: transform 0.3s ease-in-out, opacity 0.3s ease-in-out;
  transform: translateY(-100%);
  opacity: 0;
  z-index: 1000;
}

.logo {
  width: 20%;
  height: 80px;
  text-align: center;
  line-height: 80px;
  font-size: 20px;
  font-weight: 600;
  background: skyblue;
}

nav {
  width: 30%;
}

ul {
  width: 100%;
  display: flex;
  list-style: none;
  padding: 0;
  margin: 0;
}

li {
  width: 33%;
}

a {
  display: block;
  width: 100%;
  line-height: 80px;
  text-align: center;
  color: #fff;
}

.mobile-menu {
  display: none;
  width: 30px;
  height: 22px;
  text-align: center;
  cursor: pointer;
  position: absolute;
  top: 50%;
  right: 20px;
  transform: translateY(-50%);
}

.line {
  background-color: white;
  height: 4px;
  width: 100%;
  position: absolute;
}

.line:nth-of-type(1) {
  top: 0;
}

.line:nth-of-type(2) {
  top: 50%;
  transform: translateY(-50%);
}

.line:nth-of-type(3) {
  bottom: 0;
}

main {
  height: 1000px; /* ページの高さをシミュレート */
  background: #f4f4f4;
}

/* モバイル用表示切り替え */
@media (max-width: 800px) {
  .mobile-menu {
    display: block;
  }

  .pc-menu {
    display: none;
  }
}

document.addEventListener('DOMContentLoaded', function() {
  const mainHeader = document.getElementById('main-header');
  const fixedHeader = document.getElementById('fixed-header');
  let lastScrollTop = 0;

  window.addEventListener('scroll', function() {
      let scrollTop = window.scrollY || document.documentElement.scrollTop;

      if (scrollTop > lastScrollTop) {
          // スクロールダウン時
          if (scrollTop > mainHeader.clientHeight) {
              fixedHeader.style.transform = 'translateY(0)';
              fixedHeader.style.opacity = '1';
          }
      } else {
          // スクロールアップ時
          if (scrollTop <= mainHeader.clientHeight) {
              fixedHeader.style.transform = 'translateY(-100%)';
              fixedHeader.style.opacity = '0';
          }
      }
      lastScrollTop = scrollTop;
  });
});