JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://dl.dropboxusercontent.com/u/75855864/css/jquery.mCustomScrollbar.css">
<script src="https://dl.dropboxusercontent.com/u/75855864/css/jquery.mCustomScrollbar.concat.min.js"></script>
<div id="SLIDEOUT_FEED" class="SLIDEOUT_CONTAINER mCustomScrollbar" data-mcs-theme="minimal">
                                    <ul>
                                        <li>
                                            <div><img src="IMAGES/16bca5u.jpg" class="SLIDEOUT_AVATAR" /></div>
                                            <div class="notification"><b>Bigdick123</b> has requested you as friend.</div>
                                            <div class="notification_time">30 March, 6:49 AM</div>
                                        </li>
                                        <li>
                                            <div><img src="IMAGES/AvatarYouTube-Profile.jpg" class="SLIDEOUT_AVATAR" /></div>
                                            <div class="notification"><b>ezgameezlife</b> has invited you to <b>SOLOtourney, 100 cs</b>.</div>
                                            <div class="notification_time">29 March, 2:31 PM</div>
                                        </li>
                                        <li>
                                            <div><img src="IMAGES/images.jpg" class="SLIDEOUT_AVATAR" /></div>
                                            <div class="notification"><b>M0neyMak3r</b> has commented on <b>Best solo champ</b>.</div>
                                            <div class="notification_time">25 March, 7:31 PM</div>
                                        </li>
                                        <li>
                                            <div><img src="IMAGES/dts_3585279595491810.jpg" class="SLIDEOUT_AVATAR" /></div>
                                            <div class="notification"><b>AnderZEN1</b> has invited you to party.</div>
                                  ...

CSS

#SLIDEOUT_FEED {
    height: 40vh;
    background-color: blue;
}
.SLIDEOUT_CONTAINER {
    overflow: auto;
}

JavaScript

$(".SLIDEOUT_CONTAINER").mCustomScrollbar({
    scrollInertia: 100,
    mouseWheel: { 
        scrollAmount: "1vh", 
        snapAmount: "1vh",
        deltafactor: "1vh"
    },
    keyboard: { scrollAmount: "1vh" },
});