JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu-parent">


<button class="main-myButton">Каталог товаров</button>
<ul class="menu">

               
                <li><a class="first" href="">Лекарства</a>
            
                <div class="megadrop">
            
                    <div class="col">
                    <h3 >Title</h3> 
                    <ul>
                        <li><a href="#">Какие-то там лекарства</a>
                        </li>
                        <li><a href="#">Sub-menu 2</a>
                        </li>
                        <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                    </ul>
                    </div>
                    <div class="col">
                    <h3>Title</h3>
                    <ul>
                        <li><a href="#">Sub-menu 1</a>
                        </li>
                        <li><a href="#">Sub-menu 2</a>
                        </li>
                        <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
                                    <li><a href="#">Sub-menu 3</a>
                        </li>
            ...

CSS

.main-myButton{
  appearance: none;
  border: 0;
  border-radius: 10px;
  background: rgb(241, 169, 13);
  color: #fff;
  padding: 8px 16px;
  
  font-size: 24px;
  width: 300px;
  
  position: relative;
  
 /*  margin-left: 15px; */
  
}
.menu {
	 
    width: 300px;
    font-family: Verdana, Geneva, sans-serif;
    font-weight: 400;
    font-size: 15px;
    line-height: 15px;
    position: relative;
    padding: 0 0 0 4px;
    margin: 0;
    background-color: #227087;
    border: 5px solid #f1f1f1;
    background: #fff;
    border-radius: 10px;
    position:fixed;
    
}

.menu h3 {
    display: inline-block;

    margin: 20px;
    text-align: left;
    font-size: 24px;
    font-weight: normal;
    color:  #ffffff;
    background: rgb(241, 169, 13);

  }

.menu a,
.menu a:link,
.menu a:visited,
.menu a:focus,
span {
  color: black;
  text-decoration: none;
}

.menu a:hover {
 /*  color: white; */
	background: #14cc14;
  text-decoration: none;
	/* box-shadow: 0 0 0 99999px rgba(0, 0, 0, 0.1); */
}

.menu>li {
  display: block;
  text-align: left;
  margin-left: -4px;
  border-left: 1px solid rgba(255, 255, 255, 0.11);
  box-shadow: -1px 0 0 rgba(0, 0, 0, 0.1);
}

.menu>li>a {
  padding: 20px 18px;
  display: inline-block;
}

.menu>li:hover>a {
  color: white;
}

.menu>li:hover {
  background-color: #14cc14;
}

/* Megadrop width dropdown */
.menu>li>.megadrop {
  opacity: 0;
  visibility: hidden;
  position: absolute;
  list-style: none;
  top: 0px;
  left: 295px;
  width: 800px;
  min-height: 100%;
  text-align: left;
  margin-top: 30px;
  padding: 0;
  z-index: 99;
  border-bottom-left-radius: 5px;
  border-bottom-right-radius: 5px;
  overflow: hidden;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.5);
  background: #fff;
}


.menu>li:hover .megadrop {
  opacity: 1;
  visibility: visible;
  margin-top: 0px;
}

.menu ul li:hover:after {
  color: #227087;
}

.menu .col {
  width: 30%;
  float: left;
  color: white;
  margin: 0 0 0 2.2%;
}

.menu .col ul {
  padding: 0;
 ...

JavaScript

$(function() {
    $('.main-myButton').click(function() {
      $('.menu').slideToggle();
    });
  });