【CSS】ヘッダーを上部に固定し、スクロールに合わせて追従させる

<!-- ヘッダー高さ分の余白   -->
<div class="header-box"></div>
<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-box {
  height: 80px;
}

header {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  background: rgba(0, 0, 0, .1);
  z-index: 100;
  display: flex;
  justify-content: space-between;
}

.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;
}

.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: black;
  height: 4px;
  width: 100%;
  position: absolute;
  transition: all 0.3s;
}

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

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

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

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

  .pc-menu {
    display: none;
  }
}