<div class="wrapper">
  <nav>
    <div id="drop">
      <div>
        <span></span>
        <span></span>
        <span></span>
      </div>
    </div>
  </nav>
  <ul>
    <li>home</li>
    <li>about</li>
    <li>login</li>
    <li>contact</li>
    <li>privacy</li>
  </ul>
  <h1><br><br>Scroll<br>&dArr;</h1>
</div>
* {
  font-family: Segoe, "Segoe UI", "DejaVu Sans", "Trebuchet MS", Verdana,
    "sans-serif";
  font-weight: 400;
  margin: 0;
  padding: 0;
  text-align: center;
  text-transform: uppercase;
}

.hidden {
  overflow: hidden;
}

html, body {
  color: #fff;
  background-color: #1c1c1c;
}

.wrapper {
  background: linear-gradient(
    45deg,
    rgba(245, 41, 55, 1),
    rgba(124, 0, 255, 1)
  );
  width: 500px;
  height: 1200px;
  margin: 0 auto;
  overflow: hidden;
}

nav {
  width: 90%;
  height: 90px;
  padding: 0 5%;
  position: relative;
  display: flex;
  justify-content: flex-end;
  align-items: center;
  background-color: #f8f8f8;
}

nav ~ ul {
  display: flex;
  position: absolute;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  left: 1%;
  right: 1%;
  opacity: 0;
  visibility: hidden;
  z-index: 2;
  list-style: none;
  transition: 0s;
}

nav ~ ul > li {
  color: rgba(119, 0, 255, 1);
  padding: 10px;
  cursor: pointer;
  font-size: 30px;
}

nav ~ ul > li:hover {
  filter: brightness(200%);
}

nav.open ~ ul {
  transition: 0.75s ease-in-out;
  visibility: visible;
  opacity: 1;
}

#drop {
  cursor: pointer;
  position: fixed;
  background-color: #f8f8f8;
  width: 70px;
  height: 70px;
  border-radius: 0 50% 50%;
  transform: rotate(45deg);
  transition: border-radius 0.5s;
}

nav.open > #drop{
  transition: 0.5s ease-in-out;
  box-shadow: 0 0 0 1000px #f8f8f8, inset 0 0 0 20px #f8f8f8;
  position: relative;
}

#drop.active {
  border-radius: 50%;
  animation: drop 0.3s forwards;
}

#drop > div {
  transform: rotate(-45deg);
  height: 100%;
  width: 100%;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
}

#drop span {
  display: block;
  background-color: rgba(119, 0, 255, 1);
  width: 40px;
  height: 3px;
  margin: 5px 0;
  transition: 0.5s;
}

#drop span.open:nth-child(1) {
  transform: rotate(45deg) translate(5px, 5px);
}

#drop span.open:nth-child(2) {
  transform: rotate(-45deg) translate(5px, -5px);
}

#drop span.open:nth-child(3) {
  opacity: 0;
}

@keyframes drop {
  0% {
    transform: translateY(0) rotate(45deg);
  }
  50% {
    transform: translateY(30px) rotate(45deg);
  }
  100% {
    transform: translateY(10px) rotate(45deg);
  }
}
$(window).scroll(function() {
  var scroll = $(window).scrollTop();
  if (scroll > 100) 
    $("#drop").addClass("active");
  else if (scroll < 80) 
    $("#drop").removeClass("active");
});

$("#drop, nav ~ ul li").click(function() {
  $("nav, #drop span").toggleClass("open");
  $("body").toggleClass("hidden");
  $(window).scrollTop(0); //cheating
});
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External CSS

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External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js