CodePen Embed - Header Nav Overflow Into Mobile Nav

<div class="MobileNav" data-mobile-nav>
  <div class="MobileNav-overlay" data-mobile-nav-overlay></div>
  <div class="MobileNav-inner">
    <div class="MobileNav-trigger" data-mobile-nav-trigger>
      <div class="MobileNav-trigger-inner"></div>
    </div>
    <ul class="MobileNav-nav" data-mobile-nav-list>
      <li class="MobileNav-item" data-mobile-nav-item="1">
        <div class="MobileNav-item-title">
          <a href="#">About</a>
        </div>
      </li>
      <li class="MobileNav-item" data-mobile-nav-item="2">
        <div class="MobileNav-item-title">
          <a href="#">Products</a>
        </div>
      </li>
      <li class="MobileNav-item" data-mobile-nav-item="3">
        <div class="MobileNav-item-title">
          <a href="#">Services</a>
        </div>
      </li>
      <li class="MobileNav-item" data-mobile-nav-item="4">
        <div class="MobileNav-item-title">
          <a href="#">Work</a>
        </div>
      </li>
      <li class="MobileNav-item" data-mobile-nav-item="5">
        <div class="MobileNav-item-title">
          <a href="#">Contact</a>
        </div>
      </li>
    </ul>
  </div>
</div>

<header class="Header" data-header>
  <div class="Header-inner" data-header-nav>
    <div class="logo">
      <img src="https://i.imgur.com/F0k8l7N.png" alt="">
    </div>
    <nav class="Header-nav" data-nav>
      <div class="Header-nav-item" data-nav-item="1">
        <a href="#">About</a>
      </div>
      <div class="Header-nav-item" data-nav-item="2">
        <a href="#">Products</a>
      </div>
      <div class="Header-nav-item" data-nav-item="3">
        <a href="#">Services</a>
      </div>
      <div class="Header-nav-item" data-nav-item="4">
        <a href="#">Work</a>
      </div>
      <div class="Header-nav-item" data-nav-item="5">
        <a href="#">Contact</a>
      </div>
    </nav>
    <button class="MobileNav-trigger" data-mobile-nav-trigger>
      <div class="MobileNav-trigger-inner"></div>
    </button>
  </div>
</header>

<div class="container">
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?1);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?2);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?3);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?4);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?5);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?6);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?7);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?8);"></div>
  <div class="card" style="background-image: url(https://picsum.photos/1920/1080?9);"></div>
</div>

$header-transition: 0.4s ease;

body {
  align-items: center;
  background: #524fae;
  display: flex;
  justify-content: center;
  margin: 0;
  padding-top: 150px;
  width: 100vw;
}

.Header {
  align-items: center;
  background: white;
  border-bottom-left-radius: 30px;
  border-bottom-right-radius: 30px;
  display: flex;
  height: 100px;
  justify-content: center;
  position: fixed;
  text-align: center;
  top: 0;
  width: 100%;
  z-index: 10;

  &.has-mobile-button {
    .MobileNav-trigger {
      display: flex;
    }
  }

  &-inner {
    display: flex;
    justify-content: space-between;
    transition: height $header-transition;
    width: 80vw;
  }

  .logo {
    align-self: center;
    display: flex;
    height: 80px;
    margin-right: 60px;
    width: 200px;
    
    img {
      height: 100%;
      object-fit: contain;
      width: 100%;
    }
  }

  &-nav {
    display: flex;
    flex-wrap: wrap;
    height: 0;
    justify-content: space-between;
    max-height: 105px;
    max-width: 940px;
    overflow: hidden;
    width: 100%;

    &-item {
      align-items: center;
      align-self: center;
      color: darkblue;
      display: flex;
      flex-shrink: 0;
      font-family: Lato, sans-serif;
      font-size: 20px;
      font-weight: bold;
      height: 100%;
      line-height: 32px;
      margin: 0 40px;
      transition: font-size $header-transition, padding $header-transition;
      
      &:last-child {
        margin-right: 0;
      }

      a {
        color: black;
        text-decoration: none;
      }

      &:hover {
        color: darkblue !important;
      }
    }

    &:hover {
      .Header-nav-item {
        color: rgba(darkblue, 0.5);
      }
    }
  }
  
  @media screen and (min-width: 481px) {
    &-nav {
      height: auto;
    }
  }
}

.container {
  align-content: flex-start;
  display: flex;
  flex-direction: column;
  height: 100%;
  justify-content: space-around;
  width: 80vw;

  .card {
    aspect-ratio: 16 / 9;
    background: white;
    background-position: center;
    background-size: cover;
    border-radius: 20px;
    flex-shrink: 0;
    margin-bottom: 10%;
    width: 100%;
  }
}

.MobileNav {
  height: calc(100% - 55px);
  left: 0;
  pointer-events: none;
  position: fixed;
  top: 70px;
  width: 100%;
  z-index: 9;

  &-inner {
    background: white;
    display: flex;
    flex-direction: column;
    height: 100%;
    max-width: 100%;
    overflow-y: scroll;
    position: absolute;
    right: 0;
    transform: translate(0, -105%);
    transition: transform $header-transition, visibility $header-transition;
    visibility: hidden;
    width: 100%;

    .Button {
      letter-spacing: 1px;
      line-height: 3.8;
      margin: 20px -8px 20px;
      width: 100%;
    }

    .MobileNav-trigger {
      display: none;
      position: absolute;
      right: 38px;
      top: 30px;
    }
  }

  &-item {
    color: navy;
    display: none;
    font-family: Lato, sans-serif;
    font-size: 24px;
    font-weight: bold;
    margin: 26px 0;
    transition: opacity 0.3s ease;

    &.is-visible {
      display: block;
    }

    &-title {
      display: flex;
      justify-content: space-between;
      width: 100%;

      a {
        color: black;
        flex-shrink: 0;
        text-decoration: none;
      }

      span {
        min-width: 32px;
        position: relative;
        width: 100%;

        &::after {
          border-left: 2px solid lightgray;
          border-top: 2px solid lightgray;
          content: "";
          display: block;
          height: 14px;
          position: absolute;
          right: 0;
          top: 50%;
          transform: translateY(-75%) rotate(225deg);
          transition: transform 0.2s ease;
          width: 14px;
        }
      }
    }
  }

  &-overlay {
    height: 100%;
    left: 0;
    position: absolute;
    top: 0;
    transition: background $header-transition;
    width: 100%;
  }

  &-trigger {
    align-items: center;
    align-self: center;
    background: none;
    border: none;
    cursor: pointer;
    display: none;
    flex-shrink: 0;
    height: 100%;

    &-inner {
      height: 12px;
      position: relative;
      width: 24px;

      &::after {
        background: #524fae;
        content: "";
        height: 2px;
        left: 0;
        position: absolute;
        top: 0;
        transform-origin: 0% 50%;
        transition: transform 0.2s ease;
        width: 100%;
      }

      &::before {
        background: #524fae;
        bottom: 0;
        content: "";
        height: 2px;
        left: 0;
        position: absolute;
        transform-origin: 0% 50%;
        transition: transform 0.2s ease;
        width: 100%;
      }
    }

    &.is-active {
      .MobileNav-trigger-inner {
        &::after {
          transform: translateY(-4px) rotate(45deg);
        }

        &::before {
          transform: translateY(3px) rotate(-45deg);
        }
      }
    }
  }

  @media screen and (min-width: 481px) {
    height: 100%;
    top: 0;
    z-index: 11;

    &-inner {
      padding-top: 90px;
      transform: translate(100%, 0);
      width: 375px;

      .MobileNav-trigger {
        display: block;
      }
    }

    &-trigger {
      display: none;
    }
  }

  @media screen and (min-width: 871px) {
    &-inner {
      padding-top: 160px;
    }
  }
}

.is-mobilenav-open {
  overflow-y: hidden;

  .MobileNav {
    pointer-events: all;

    &-inner {
      transform: translate(0, 0);
      visibility: visible;
    }

    &-overlay {
      background: rgba(0, 0, 0, 0.25);
    }
  }
}

View Compiled

function isOverflown({ clientWidth, clientHeight, scrollWidth, scrollHeight }) {
  return scrollHeight > clientHeight || scrollWidth > clientWidth;
}

function isVisible(parent, child) {
  return !(
    (child.offsetLeft - parent.offsetLeft >= parent.offsetWidth)
    || (child.offsetTop - parent.offsetTop >= parent.offsetHeight)
  );
}

function init() {
  const page = document.querySelector('[data-main-page]');
  const header = document.querySelector('[data-header]');
  const topbar = document.querySelector('[data-topbar]');
  const nav = header.querySelector('[data-nav]');
  const navItems = nav.querySelectorAll('[data-nav-item]');
  const mobileNavList = document.querySelector('[data-mobile-nav-list]');
  const mobileNavItems = document.querySelectorAll('[data-mobile-nav-item]');
  const mobileNavTriggers = document.querySelectorAll('[data-mobile-nav-trigger]');
  const mobileNavOverlay = document.querySelector('[data-mobile-nav-overlay]');

  // Resize Observer checking whether to show mobile nav button based on if a nav element is overflowing
  const showMobileNavButton = () => {
    const navHidden = getComputedStyle(nav, null).display === 'none';
    if (navHidden) {
      mobileNavItems.forEach((item) => {
        item.classList.add('is-visible');
      });
    }

    const resizeObserver = new window.ResizeObserver((entries) => {
      for (const entry of entries) {
        header.classList.toggle('has-mobile-button', isOverflown(nav));
        navItems.forEach((item) => {
          const navItems = Array.from(mobileNavItems);
          const matchingNavItem = navItems.find(el => el.dataset.mobileNavItem === item.dataset.navItem);

          matchingNavItem.classList.toggle('is-visible', !isVisible(nav, item));
        });
      }
    });

    resizeObserver.observe(nav);
  };

  // Mobile nav button open/close
  mobileNavTriggers.forEach((trigger) => {
    trigger.addEventListener('click', () => {
      mobileNavTriggers.forEach((trigger) => trigger.classList.toggle('is-active'));
      document.body.classList.toggle('is-mobilenav-open');
    });
  });

  // Mobile nav overlay close
  mobileNavOverlay.addEventListener('click', () => {
    mobileNavTriggers.forEach((trigger) => {
      trigger.classList.remove('is-active');
    });
    document.body.classList.remove('is-mobilenav-open');
  });

  showMobileNavButton();
}

init();

This Pen is owned by Chris Weissenberger on CodePen.

See more by @CAWeissen on CodePen

External CSS

https://fonts.googleapis.com/css?family=Lato&display=swap

External JavaScript

https://cdn.jsdelivr.net/npm/resize-observer-polyfill@1.5.1/dist/ResizeObserver.min.js