Попробуйте, во фрагменте
nav{
display:flex;
justify-content: space-between;
align-items: center;
padding:0px 10px;
}
nav a{
text-decoration: none;
}
.menu{
display:flex;
justify-content: space-between;
}
#menuBtn{
padding:10px;
}
.hiddenlayer{
background-color: rgba(100%,100%,100%,0.9);
box-shadow: 0px 2px 10px rgba(0,0,0,0.2);
position:absolute;
max-height: 0vh;
overflow: hidden;
transition: 1s;
}
.hiddenlayer>div{
padding:10px;
}
#menuBtn:hover .hiddenlayer{
max-height: 100vh;
}
<nav>
<big>LOGO</big>
<div class="menu">
<div id="menuBtn">ABOUT</div>
<div id="menuBtn">CATEGORY
<div class="hiddenlayer">
<div>
<a href="#">CATEGORY 1</a>
</div>
<div class="dropdown megamenu">
<a href="#">CATEGORY 1</a>
</div>
<div class="dropdown tt-megamenu-col-01">
<a href="#">CATEGORY 1</a>
</div>
<div class="dropdown tt-megamenu-col-01">
<a href="#">CATEGORY 1</a>
</div>
</div>
</div>
<div id="menuBtn">CONTACT</div>
</div>
</nav>