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CSS

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HTML 

              
                <h1>

  Flex Menu</h1>
<div class="container">
  <div class="flex-box"></div>
  <div class="flex-box"></div>
  <div class="flex-box"></div>

  <div class="menu">
    <ul class="menu__list">
      <li class="menu__item">Home</li>
      <li class="menu__item">About</li>
      <li class="menu__item dropdown-holder dropdown-holder1">
        Widgets
        <div class="hover-box">
          <ul class="hover-box__list">
            <li class="hover-box__item">Big Widget</li>
            <li class="hover-box__item">Bigger Widgets</li>
            <li class="hover-box__item">Huge Widget</li>
          </ul>
        </div>

      </li>
      <li class="menu__item dropdown-holder">Kabobs
        <div class="hover-box">
          <ul class="hover-box__list">
            <li class="hover-box__item">Shiskabobs</li>
            <li class="hover-box__item">BBQ kabobs</li>
            <li class="hover-box__item">Summer Kabobs</li>
          </ul>
        </div>

      </li>
      <li class="menu__item">Contact</li>

    </ul>
  </div>

  <div class="toggle">
    <span class="toggle__span"></span>
    <span class="toggle__span"></span>
    <span class="toggle__span"></span>
  </div>

</div>
!

CSS 

              
                * {
  margin: 0;
  padding: 0;
  font-family: "Work Sans", sans-serif;
}

h1 {
  position: absolute;
  top: 50%;
  transform: translate(0, -50%);
  color: #fff;
  font-weight: 700;
  font-size: 48px;
  z-index: 10;
  width: 100%;
  text-align: center;
  @media only screen and (max-width: 880px){
    font-size: 32px;
  }
}

.container {
  display: flex;
  height: 100vh;
  z-index: 5;
  &__open {
    .menu {
      display: block;
    }
  }
}

.container__open .toggle {
  &__span {
    &:nth-child(1) {
      top: 18px;
      transform: rotate(45deg);
    }
    &:nth-child(2) {
      background-color: transparent;
    }
    &:nth-child(3) {
      top: 18px;
      transform: rotate(-45deg);
    }
  }
}

.container__open .flex-box:nth-child(2) {
  flex: 0 0 0;
}

.flex-box {
  background-color: #004d40;
  flex: 0 1 50%;
  transition: 0.5s all ease-in-out;
  position: relative;
  z-index: 15;
}

.flex-box:nth-child(2) {
  flex: 1 0 100%;
  background-color: #333;
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 56px;
  color: #fff;
  font-weight: 700;
  z-index: 1;
}

.container:hover.flex-box:nth-child(2) {
  flex: 0 0 0;
}

.menu {
  display: none;
  width: 100%;
  position: fixed;
  top: 0;
  left: 0%;
  height: 100vh;
  color: #fff;
  transform: translateY(-100%);
  background-color: transparent;
  animation: 0.4s slideDown 0.25s forwards;
  z-index: 100;
  -webkit-backface-visibility: hidden;

  &__list {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    margin: 0 auto;
    display: flex;
    align-items: center;
    justify-content: center;
    list-style: none;
    padding: 0;
    @media only screen and (max-width: 880px) {
      flex-direction: column;
    }
  }
  &__item {
    font-weight: 700;
    font-size: 44px;
    padding: 1rem;
    margin: 0 1rem;
    text-align: center;
    background: linear-gradient(
      120deg,
      transparent 0%,
      transparent 50%,
      #fff 50%
    );
    background-size: 240%;
    transition: 0.3s all ease-in-out;
    cursor: pointer;
    position: relative;
    box-sizing: border-box;
    -webkit-backface-visibility: hidden;

    &:hover {
      background-position: 99%;
      color: #004d40;
    }

    @media only screen and(max-width: 1250px) {
      font-size: 36px;
    }

    @media only screen and (max-width: 1000px) {
      font-size: 28px;
    }
  }
}

.hover-box {
  background-color: #fff;
  color: green;
  display: none;
  animation: 0.6s fadeIn ease-in-out;
  position: absolute;
  color: #004d40;
  top: 100%;
  width: 100%;
  left: 0;
  border-top: 4px solid #004d40;
  backface-visibility: none;

  &__list {
    list-style: none;
    margin: 0;
    padding: 0;
    display: flex;
    flex-direction: column;
    align-items: stretch;
    width: 100%;
  }
  &__item {
    box-sizing: border-box;
    width: 100%;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 20px;
    padding: 6px 12px;
    border-bottom: 2px solid #004d40;
    transition: 0.2s background;
    box-sizing: border-box;
    backface-visibility: none;
    &:hover {
      background-color: #a5d6a7;
    }
  }
  @media only screen and (max-width: 880px) {
    display: block;
    position: relative;
    width: auto;
    border-top: 2px solid #004d40;

    &__list {
      flex-direction: row;
    }
  }
}

.toggle {
  cursor: pointer;
  position: fixed;
  top: 2rem;
  left: 2rem;
  z-index: 10000;
  width: 40px;
  height: 40px;

  &__span {
    height: 3px;
    width: 100%;
    background-color: #fff;
    position: absolute;
    transition: 0.4s;
    &:nth-child(1) {
      top: 10px;
    }
    &:nth-child(2) {
      top: 19px;
    }
    &:nth-child(3) {
      top: 28px;
    }
  }
}

@keyframes slideDown {
  from {
    transform: translateY(-100%);
  }
  to {
    transform: translateY(0);
  }
}
!

JS 

              
                // MUST REFRESH PAGE ON DIFFERENT VIEWPORT FOR HOVER EFFECTS

if ($(window).width() > 739) {
  $(".dropdown-holder").on("mouseenter", function(e) {
    $(".hover-box", this).slideDown("fast");
    $(".dropdown-holder").on("mouseleave", function(e) {
      $(".hover-box", this).slideUp("fast");
    });
  });
}

document.querySelector(".toggle").addEventListener("click", function() {
  document.querySelector(".container").classList.toggle("container__open");
});
!
999px

Console