JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menutop" class="menu">
			<nav class="navbar navbar-expand-sm p-0 m-0" aria-label="navbar">
			<div class="container-fluid p-0 m-0 align-items-start">
                
				<button class="navbar-toggler navbar-dark m-2" type="button" data-bs-toggle="collapse" data-bs-target="#navbarTop" aria-controls="navbarTop" aria-expanded="false" aria-label="Toggle navigation">
					<span class="navbar-toggler-icon"></span>
				</button>

				<div id="navbarTop" class="collapse navbar-collapse justify-content-end p-1 m-0">
					<ul class="nav navbar-nav px-3">
						<li class="nav-item">
							<a class="scrollTo" href="#section1">INTRO</a>
						</li>
						<li class="nav-item">
							<a class="scrollTo" href="#section2">SITES</a>
						</li>
						<li class="nav-item">
							<a class="scrollTo" href="#section3">LDDC</a>
						</li>
						<li class="nav-item">
							<a class="scrollTo" href="#section4">VIDEOS</a>
						</li>
						<li class="nav-item">
							<a class="scrollTo" href="#section5">PHOTOS</a>
						</li>
						<li class="nav-item">
							<a class="scrollTo" href="#section6">ME</a>
						</li>
					</ul>
				</div>

				</div>
			</nav>
		</div>

CSS

body {
  background:red;
}
/* nav */
#menutop nav .navbar-nav {
	margin:0 10px 0 0 !important;
	padding:0 !important;
	font-size: 0.8em;
}
#menutop nav .navbar-nav a {
	display: block;
	color: white;
	padding: 10px 6px 10px 0;
	line-height: 1.3em !important;
	font-family: 'Raleway', sans-serif;
	text-align:right;
	font-size: 1.3em;
}
#menutop nav .navbar-nav a:hover {
	color: white;
	filter: brightness(1.75);
}

nav li {
    display: inline-block;
    overflow:hidden;
    vertical-align: middle;
}
nav ul li:last-child {
    margin-right:0;
}
nav li a{
   display:block;
   padding: 8px 0;
   position:relative;
   font-size:1em;
   transition:font-size 0.2s;
}
nav li a::before {
    content:"";
    width:0;
    height:2px;
    background: 
white;
    position:absolute;
    bottom:0;
    left:50%;
    right:0;
    transition:all 0.2s;
}
nav li a:hover::before {
    min-width:100%;
    left:0;
}

/* ------------ */
/* Transitions and class for reduced height */
#menutop,
#menutop .navbar-brand,
#menutop .navbar-brand img,
#menutop nav .navbar-nav,
#menutop nav .navbar-nav a,
#menutop.menu-shrink {
	-webkit-transition: all 0.3s; -moz-transition: all 0.3s; transition: all 0.3s;
}
/* ------------ */
/* menu-shrink */
#menutop.menu-shrink {
	height: 80px;
}
#menutop.menu-shrink:hover {
	background-color: rgba(255,255,255,0.5);
}
#menutop.menu-shrink .navbar-brand {
	width: 75px;
	padding-left: 4px;
	padding-top: 4px;
}
#menutop nav .navbar-nav {
	padding-top:15px !important;
}
#menutop.menu-shrink nav .navbar-nav a {
	font-size: 1.3em;
	color: black;
}

JavaScript

$(function() {
    var menutop = $("#menutop");
    // au chargement / actualisation de la page
        if ($(window).scrollTop() >= 200) {
            menutop.addClass("menu-shrink");
        } else {
            menutop.removeClass("menu-shrink");
        }
    // au scroll
    $(window).scroll(function() {    
        if ($(window).scrollTop() >= 200) {
            menutop.addClass("menu-shrink");
        } else {
            menutop.removeClass("menu-shrink");
        }
    });
});