html

  <header class="common__header js--gnav">
    <div class="inner-layout">
      <h1 class="headerlogo">
        <a href="/">ロゴ</a>
      </h1>
    </div>
    <div class="gnav__hamburger js--hamburger js--is--open">
      <div class="border"></div>
    </div>
    <nav class="gnav__layout">
      <ul>
        <li>
          <a href="">メニュー1</a>
        </li>
        <li>
          <a href="">メニュー2</a>
        </li>
        <li>
          <a href="">メニュー3</a>
        </li>
        <li>
          <a href="">メニュー4</a>
        </li>
      </ul>
    </nav>
    <div class="gnav__bg js--is--close"></div>
  </header>

scss

// .common__header
//=====================================================
.common__header {
  width: 100%;
  background: colors(white);
  @include z-index-3;
  @include media(pc) {
    position: fixed;
    top: 0;
    left: 0;
  }
  .inner-layout {
    position: fixed;
    top: 0;
    left: 0;
    @include z-index-1;
    height: 52px;
    width: calc(100% - 52px);
    margin: 0 auto;
    display: flex;
    align-items: center;
    justify-content: space-between;
    @include media(pc) {
      position: static;
      width: 87.8rem;
      height: auto;
      margin: 0 auto;
      background: yellow;
    }
  }
  .headerlogo {
    margin-left: 18px;
    font-size: 1.4rem;
    font-weight: bold;
    line-height: 1.25;
    @include media(pc) {
      margin-left: 0;
      font-size: 1.8rem;
    }
    a {
      color: colors(primary);
    }
  }
}

// .gnav__wrap
//=====================================================
.js--gnav {
  transition: 0.3s ease;
  &.is--gnavopen {
    background: none;
  }
}

// .gnav__hamburger
//=====================================================
.gnav__hamburger {
  width: 52px;
  height: 52px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  position: fixed;
  top: 0;
  right: 0;
  @include z-index-3;
  background: colors(primary);
  gap: 5px 0;
  @include media(pc) {
    display: none;
  }
  &:before {
    content: "";
    display: block;
    width: 50%;
    height: 1px;
    background: rgba(255,255,255,0.85);
    transition: 0.3s ease;
  }
  &:after {
    content: "";
    display: block;
    width: 50%;
    height: 1px;
    background: rgba(255,255,255,0.85);
    transition: 0.3s ease;
  }
  .border {
    width: 50%;
    height: 1px;
    background: rgba(255,255,255,0.85);
    transition: 0.3s ease;
  }
  // 別パターン
  &.js--is--close {
    &:before {
      width: 40%;
      transform: rotate(45deg) translateX(3px) translateY(3px);
    }
    &:after {
      width: 40%;
      transform: rotate(-45deg) translateX(6px) translateY(-5px);
    }
    .border {
      opacity: 0;
    }
  }
}

// .gnav__layout
//=====================================================
.gnav__layout {
  position: fixed;
  @include z-index-3;
  top: 50%;
  left: 50%;
  transform: translateX(-50%) translateY(-50%);
  display: none;
  @include media(pc) {
    display: block;
    position: static;
    transform: none;
  }
  ul {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    font-size: 1.6rem;
    text-align: center;
    @include media(pc) {
      flex-direction: row;
    }
    li {
      margin-bottom: 1.85em;
      line-height: 1.25;
      @include media(pc) {
        margin-bottom: 0;
      }
      &:last-of-type {
        margin-bottom: 0;
      }
      a {
        white-space: nowrap;
        color: colors(primary);
        transition: 0.3s ease;
      }
    }
  }
}

// .gnav__bg
//=====================================================
.gnav__bg {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100vh;
  height: 100dvh;
  @include z-index-2;
  display: none;
  background: rgba(255, 255, 255, 0.7);
  -webkit-backdrop-filter: blur(7.5px);
  backdrop-filter: blur(7.5px);
}

js

// g-nav modal
//=====================================================
$(function() {

  var scrollPos; //topからのスクロール位置
  $('body').on( 'click', '.js--hamburger.js--is--open', function() {
    scrollPos = $(window).scrollTop(); //topからのスクロール位置を格納
    $('.js--gnav').addClass('is--gnavopen');
    $('.js--hamburger').removeClass('js--is--open').addClass('js--is--close');
    $('.gnav__bg, .gnav__layout, .gnav__colosebg').fadeIn(450);
    $('body').addClass('fixed').css({ top: -scrollPos }); //背景固定
    return false;//<a>を無効化
  });

  $('body').on( 'click', '.js--is--close', function() {
    $('.js--hamburger').removeClass('js--is--close').addClass('js--is--open');
    $('.gnav__bg, .gnav__layout, .gnav__colosebg').fadeOut(450);
    $('.js--gnav').removeClass('is--gnavopen');
    $('body').removeClass('fixed').css({ top: 0 });//背景固定を解除
    $(window).scrollTop(scrollPos);//元の位置までスクロール
  });

  // ページ内リンク対策
  //$('body').on( 'click', '.js-modal-nav-close-a', function() {
    //$('.modal-nav').fadeOut();//モーダルをフェードアウト
    //$('.modal-nav').addClass('modal-nav__fadeout');
    //$('.modal-nav').removeClass('modal-nav__fadein');
    //$('body').removeClass('fixed').css({ top: 0 });//背景固定を解除
    //$('.js-ic_menu').removeClass('none');//ハンバーガーアイコン表示
    //$('.js-modal-nav-close').addClass('js-modal-nav-open');
    //$('.js-modal-nav-close').removeClass('js-modal-nav-close');
  //});

});