ハンバーガーメニューjQuery実装について

ハンバーガーメニューをjQeuryで実装中なのですが、ハンバーガーメニューをクリックしてもfadeToggleメソッドで表示されません。また、display:noneせず表示したときに要素の上下関係がおかしくなり、見出しがメニューより上にきてしまいます。
この二点をどうかご教授ください。

コード
```HTML
<!doctype html>
<html lang="ja">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <!-- Bootstrap CSS -->
    <!-- <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css" integrity="sha384-9aIt2nRpC12Uk9gS9baDl411NQApFmC26EwAOH8WgZl5MYYxFfc+NcPb1dKGj7Sk" crossorigin="anonymous"> -->
    <link rel="stylesheet" href="css/reset.css">
    <link rel="stylesheet" type="text/css" href="css/style.css">
    <link href="https://use.fontawesome.com/releases/v5.15.1/css/all.css" rel="stylesheet">
    <title>Shiro</title>
  </head>
  <body>

  <div class="bg">
    <header class="header">
      <div class="container header-inner">
        <h1 class="header-logo"><a href="#">Shiro</a></h1>
        <div class="burger-btn"><!-- ③ハンバーガーボタン -->
          <span class="bar bar_top"></span>
          <span class="bar bar_mid"></span>
          <span class="bar bar_bottom"></span>
        </div>
        </div>
        <nav class="header-nav">
              <ul class="header-menu">
                <li><a href="#">Concept</a></li>
                <li><a href="#">Work</a></li>
                <li><a href="#">Florist</a></li>
                <li><a href="#">Shopinfo</a></li>
                <li><a href="#">Contact us</a></li>
              </ul>
            </nav>
      </div>
      
    </header>

    <div class="mv">
      <h2 class="mv-ttl">Shiro</h2>
      <p class="mv-subttl">- White flower shop -</p>
    </div>
  </div>


css
body {
  font-family: "Helvetica Neue", Arial, "Hiragino Kaku Gothic ProN", "Hiragino Sans", Meiryo, sans-serif;
}

a {
  text-decoration: none;
  color: #333;
}

img {
  max-width: 100%;
  height: auto;
}

.container {
  max-width: 960px;
  margin: 0 auto;
}

.bg {
  height: 620px;
  background: linear-gradient(rgba(0, 0, 0, 0.2), rgba(0, 0, 0, 0.2)), url(../img/header.png);
  background-size: cover;
  position: relative;
  margin-bottom: 80px;
}

@media (max-width: 768px) {
  .txt-br {
    display: none;
  }
}

.padd-120 {
  margin-top: 120px;
}

@media (max-width: 768px) {
  .padd-120 {
    margin-top: 0;
  }
}

.header {
  height: 64px;
  padding-top: 40px;
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
}

.header-logo a {
  font-size: 36px;
  color: #fff;
}

.header-inner {
  height: 100%;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.header-inner i {
  color: white;
}

@media (max-width: 768px) {
  .header-inner {
    width: 90%;
  }
}

.header-nav {
  display: none;
  position: fixed;
  background-color: white;
  text-align: center;
  color: #000;
  font-size: 18px;
  line-height: 11vh;
  z-index: 99;
  opacity: 0.8;
  height: calc(100vh - 64px);
  width: 100vw;
}

.header li {
  border: solid 1px #ccc;
}

.header-menu a:hover {
  text-decoration: underline;
}

.burger-btn {
  display: block;
  width: 39px;
  height: 39px;
  position: relative;
  z-index: 3;
  border: none;
}

.bar {
  width: 20px;
  height: 1px;
  display: block;
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
  background-color: #fff;
}

.bar_top {
  top: 10px;
}

.bar_mid {
  top: 50%;
  transform: translate(-50%, -50%);
}

.bar_bottom {
  bottom: 10px;
}

.burger-btn.close .bar_top {
  transform: translate(-50%, 10px) rotate(45deg);
  transition: transform .3s;
}

.burger-btn.close .bar_mid {
  opacity: 0;
  transition: opacity .3s;
}

.burger-btn.close .bar_bottom {
  transform: translate(-50%, -8px) rotate(-45deg);
  transition: transform .3s;
}

.mv {
  color: #fff;
  position: absolute;
  z-index: 0;
  bottom: 0;
  top: 50%;
  left: 50%;
  -webkit-transform: translate(-50%, -30%);
  transform: translate(-50%, -30%);
}

.mv-ttl {
  font-size: 120px;
  line-height: 1.4;
}

@media (max-width: 768px) {
  .mv-ttl {
    font-size: 110px;
  }
}

.mv-subttl {
  font-size: 16px;
  text-align: center;
}

.concept {
  text-align: center;
  width: 100%;
  background: linear-gradient(#fff 70%, #ededed 30%);
  padding-bottom: 80px;
  margin-bottom: 80px;
}

@media (max-width: 768px) {
  .concept {
    margin-bottom: 40px;
  }
  .concept .container {
    width: 90%;
    margin: 0 auto;
  }
  .concept-img img {
    object-fit: cover;
    height: 400px;
    margin-bottom: 16px;
  }
}

.concept-ttl {
  font-size: 36px;
  margin-bottom: 32px;
  padding-bottom: 24px;
}

.concept-ttl::after {
  content: '';
  position: absolute;
  bottom: 0;
  left: 50%;
  width: 40px;
  height: 1px;
  background: #2b2b2b;
  transform: translateX(-50%);
}

.concept-txt {
  font-size: 16px;
  line-height: 1.4;
  margin-bottom: 40px;
}

@media (max-width: 768px) {
  .concept-txt {
    text-align: left;
  }
}

@media (max-width: 768px) {
  .concept-txt-br {
    display: none;
  }
}

.work {
  text-align: center;
  width: 100%;
}

@media (max-width: 768px) {
  .work {
    padding: 0;
    width: 90%;
    margin: 0 auto;
  }
}

.work-ttl {
  font-size: 36px;
  position: relative;
  margin-bottom: 32px;
  padding-bottom: 24px;
}

.work-ttl::after {
  content: '';
  position: absolute;
  bottom: 0;
  left: 50%;
  width: 40px;
  height: 1px;
  background: #2b2b2b;
  transform: translateX(-50%);
}

.work-txt {
  font-size: 16px;
  line-height: 1.4;
  margin-bottom: 40px;
}

@media (max-width: 768px) {
  .work-txt {
    text-align: left;
  }
}

.work-contents-box {
  display: flex;
  flex-wrap: wrap;
  flex-direction: column;
  align-content: space-between;
  height: 1680px;
}

@media (max-width: 768px) {
  .work-contents-box {
    flex-direction: row;
    margin-bottom: 20px;
    max-width: 400px;
    margin-left: auto;
    margin-right: auto;
    height: auto;
  }
}

.work-content-box {
  width: 420px;
}

.work-box {
  margin-bottom: 80px;
}

.work-box-ttl {
  font-size: 24px;
  font-weight: bold;
  line-height: 1.7;
}

.work-box-txt {
  line-height: 1.4;
}

.florist {
  text-align: center;
  width: 100%;
  background: linear-gradient(#ededed 60%, #fff 40%);
  padding-top: 40px;
  margin-bottom: 120px;
}

@media (max-width: 768px) {
  .florist {
    width: 90%;
    margin-left: auto;
    margin-right: auto;
  }
}

.florist-ttl {
  font-size: 36px;
  position: relative;
  margin-bottom: 32px;
  padding-bottom: 24px;
}

.florist-ttl::after {
  content: '';
  position: absolute;
  bottom: 0;
  left: 50%;
  width: 40px;
  height: 1px;
  background: #2b2b2b;
  transform: translateX(-50%);
}

.florist-txt {
  font-size: 16px;
  line-height: 1.4;
  margin-bottom: 40px;
}

.florist-name {
  font-size: 24px;
  font-weight: bold;
  line-height: 1.7;
}

.florist-content-box {
  width: calc(33.33333333% - 25px * 2 / 3);
  margin-right: 30px;
  text-align: left;
}

@media (max-width: 768px) {
  .florist-content-box {
    width: 100%;
    text-align: center;
    margin-right: auto;
    margin-left: auto;
  }
}

.florist-contents-box {
  display: flex;
}

@media (max-width: 768px) {
  .florist-contents-box {
    flex-direction: column;
  }
}





.headerColorScroll {
  background-color: white;
  box-shadow: 0 3px 6px rgba(0, 0, 0, 0.16);
  z-index: 999;
}


js
$(function () {
    $(window).on("scroll", function () {
      const sliderHeight = 640;
      if (sliderHeight - 30 < $(this).scrollTop()) {
        $(".header").addClass("headerColorScroll").css('padding-top','0');
        $(".header-logo a").css("color",'#333');
        $(".menu-btn i").css("color",'#333');
      } else {
        $(".header").removeClass("headerColorScroll");
        $(".header-logo a").css("color",'#fff');
        $(".menu-btn i").css("color",'#fff');
      }
    });
  });

  $('.burger-btn').on('click',function(){
    $('.burger-btn').toggleClass('close');
    $('.header-menu').fadeToggle(500);
  });