After experimenting with your fiddler tool, I encountered the issue.
However, upon transferring this code to an HTML file, it appears to be functioning correctly.
I am sharing it here as an answer because it simplifies the process of sharing code.
Feel free to copy and paste this code into an HTML document for testing purposes. It seems that there might have been a restriction within the fiddler tool. Please update us on your findings.
<html>
<head>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/malihu-custom-scrollbar-plugin/3.1.5/jquery.mCustomScrollbar.css">
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/jquery-nice-select/1.1.0/css/nice-select.min.css">
<style>
body {
overflow: hidden;
}
.wrapper {
height: 100vh;
}
.mCSB_dragger_bar {
background-color: #988f01 !important;
}
.nice-select {
float: none;
.list {
width: 100%;
}
}
</style>
</head>
<body>
<div class="wrapper">
<p>
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Dolore, porro voluptatibus accusantium saepe dolor, numquam, debitis magnam consequatur ullam quaerat laudantium pariatur suscipit, ratione odio libero dicta deleniti! Adipisci, vero.
</p>
[Rest of the HTML code has been omitted for brevity]
</div>
<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-nice-select/1.1.0/js/jquery.nice-select.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/malihu-custom-scrollbar-plugin/3.1.5/jquery.mCustomScrollbar.concat.min.js"></script>
[JavaScript code for functionality omitted for brevity]
</body>
</html>