途中でナビの高さが半分

デモ

See the Pen nav 20 by logsuke (@design-list) on CodePen.

コード

<!-- Boxicons headタグ内へ -->
<link href='https://unpkg.com/boxicons@2.1.2/css/boxicons.min.css' rel='stylesheet'>
     <header class="header" id="header">
          <nav id="nav" class="nav">
               <a href="#" class="logo">
                    <svg viewBox="0 0 496 512" width="100" title="meh-rolling-eyes">
                         <path
                              d="M248 8C111 8 0 119 0 256s111 248 248 248 248-111 248-248S385 8 248 8zM88 224c0-24.3 13.7-45.2 33.6-56-.7 2.6-1.6 5.2-1.6 8 0 17.7 14.3 32 32 32s32-14.3 32-32c0-2.8-.9-5.4-1.6-8 19.9 10.8 33.6 31.7 33.6 56 0 35.3-28.7 64-64 64s-64-28.7-64-64zm224 176H184c-21.2 0-21.2-32 0-32h128c21.2 0 21.2 32 0 32zm32-112c-35.3 0-64-28.7-64-64 0-24.3 13.7-45.2 33.6-56-.7 2.6-1.6 5.2-1.6 8 0 17.7 14.3 32 32 32s32-14.3 32-32c0-2.8-.9-5.4-1.6-8 19.9 10.8 33.6 31.7 33.6 56 0 35.3-28.7 64-64 64z" />
                    </svg>
                    logo
               </a>
               <div class="nav-menu" id="navMenu">
                    <ul class="nav-links">
                         <li><a href="#section1" class="active nav-link section1">Section1</a></li>
                         <li><a href="#section2" class="nav-link section2">Section2</a></li>
                         <li><a href="#section3" class="nav-link section3">Section3</a></li>
                         <li><a href="#section4" class="nav-link section4">Section4</a></li>
                    </ul>
               </div>

               <div class="nav-toggle" id="navToggle">
                    <i id="navIcon" class='bx bx-menu'></i>
               </div>
          </nav>
     </header>
     <main>
          <section id="section1" class="section">
               <div class="section-title">
                    <h2>Section1</h2>
               </div>
          </section>
          <section id="section2" class="section">
               <div class="section-title">
                    <h2>Section2</h2>
               </div>
          </section>
          <section id="section3" class="section">
               <div class="section-title">
                    <h2>Section3</h2>
               </div>
          </section>
          <section id="section4" class="section">
               <div class="section-title">
                    <h2>Section4</h2>
               </div>
          </section>

     </main>

/*=== Base ===*/

* {
     margin: 0;
     padding: 0;
     box-sizing: border-box
}

:root {
     --nav-height: 8rem;
     --max-width: 1024px;
     --container-padding: 0 4rem;
     --gap: 2rem;
     --bg-color: rgb(240, 240, 240);
}

/*=== Font ratio ===*/

html {
     scroll-behavior: smooth;
     font-size: 62.5%;
}

body {
     font-size: 1.6rem;
}

ul {
     list-style: none;
}

a {
     text-decoration: none;
}

/* header */
header {
     width: 100%;
     position: fixed;
     top: 0;
     left: 0;
     z-index: 100;
     background-color: var(--bg-color);
}

.nav svg {
     width: 25px;
     padding: 2px;
     margin-right: 5px;
}

/* nav */
.nav {
     width: 100%;
     max-width: var(--max-width);
     height: var(--nav-height);
     display: flex;
     justify-content: space-between;
     align-items: center;
     margin: 0 auto;
     padding: 0 3rem;
     transition: height .3s;
}

.nav.harf {
     height: calc(var(--nav-height) / 2);
}

.logo,
.nav-toggle {
     color: var(--text-color);
     cursor: pointer;
}

.logo {
     font-weight: bold;
     font-size: 2rem;
     z-index: 100;
     display: flex;
     justify-content: center;
     align-items: center;
}

.nav-toggle {
     font-size: 3rem;
     display: flex;
     align-items: center;
     z-index: 200;
}

.nav-toggle i {
     display: none;
}

.nav-links {
     display: flex;
     align-items: center;
     justify-content: center;
     gap: 2rem;
}

.nav-links a {
     color: var(--text-color);
     position: relative;
}

.nav-links a.active::before {
     content: '';
     position: absolute;
     bottom: -.8rem;
     left: 0;
     width: 100%;
     height: 3px;
     border-radius: 3px;
     background: rgb(147, 147, 247);
}

/*スマホサイズナビゲーションバー*/
@media screen and (max-width: 768px) {

     .nav-menu {
          position: fixed;
          top: 0;
          right: -50%;
          background: var(--bg-color);
          box-shadow: var(--shadow);
          width: 50%;
          height: 100vh;
          padding-top: var(--nav-height);
          transition: transform .5s ease-in-out;
          z-index: 120;
     }

     .nav-menu.toggle {
          transform: translateX(-100%);
     }

     .nav-toggle i {
          display: block;
     }

     .nav-links {
          flex-direction: column;
     }

     .nav-links li {
          height: var(--nav-height);
          width: 100%;
          display: flex;
          justify-content: center;
          align-items: center;
     }
}

/*=== section ===*/

.section {
     height: calc(100vh - var(--nav-height));
     padding-top: var(--nav-height);
}

.section-title {
     display: flex;
     justify-content: center;
     margin: 5rem 0;
     font-size: 2.5rem;
     position: relative;
}

.section-title h2 {
     position: relative;
     font-size: 2.5rem;
}

#section1 {
     background: rgb(219, 199, 119);
}
#section2 {
     background: rgb(128, 217, 177);
}
#section3 {
     background: rgb(210, 100, 100);
}
#section4 {
     background: rgb(51, 36, 186);
}

const nav = document.getElementById('nav'),
    navMenu = document.getElementById('navMenu'),
    navToggle = document.getElementById('navToggle'),
    navIcon = document.getElementById('navIcon'),
    navLinks = document.querySelectorAll('.nav-link')

navToggle?.addEventListener('click', function () {
    if (navIcon.classList.contains('bx-menu')) {
        navIcon.classList.replace('bx-menu', 'bx-x');
        navMenu.classList.toggle('toggle');
    } else {
        navIcon.classList.replace('bx-x', 'bx-menu');
        navMenu.classList.toggle('toggle');
    }
})

navLinks.forEach(function (li) {
    li.addEventListener('click', function () {
        navIcon.classList.replace('bx-x', 'bx-menu');
        navMenu.classList.remove('toggle');
    })
})

const options = {
    // root: null,
    // rootMargin: "-50% 0px",
    threshold: 0.2
    // threshold: [0.2, 1.0]
};
const sections = document.querySelectorAll('section');
const observer = new IntersectionObserver(intersect, options);

sections.forEach(function (section) {
    observer.observe(section);
})

function intersect(entries) {
    entries.forEach(entry => {
        if (entry.isIntersecting) {
            // console.log(entry.target.id, 'が画面内に入った');
            if (entry.target.id == 'section1') {
                nav.classList.remove('harf');
            }
        }

        if (!entry.isIntersecting) {
            // console.log(entry.target.id, 'が画面から出た');
            if (entry.target.id == 'section1') {
                nav.classList.add('harf');
            }
        }
        
        if (entry.intersectionRatio === 1) {
            // console.log(entry.target.id, 'が画面に完全に収まった');
        }
    });
}