JSFiddle - React, Tailwind, and code Playground

HTML

<div class="scroll-bar-wrap">
  <div class="scroll-box">
    <div class="main">
      <h1>Lorem ipsum dolor sit amet
      </h1>
      <img src="http://placekitten.com/600/400" />
      <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut
        labore
        et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea
        rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem
        ipsum
        dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et
        dolore
        magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum.
        Stet
        clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit
        amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna
        aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet
        clita
        kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet,
        consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore
        et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea
        rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem
        ipsum
        dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et
        dolore
        magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum.
        Stet
        clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit
        amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna
        aliquyam erat,...

CSS

body,
html {
  margin: 0;
  padding: 0;
  font-family: monospace;
}

.main {
  padding: 20px;
}

h1 {
  font-size: 50px;
  margin: 0;
}

p {
  font-size: 12px;
  margin: 0;
}

img {
  display: block;
  margin: 0 auto;
  padding: 20px 0px;
  box-sizing: border-box;
  max-width: 100%;
}

.scroll-bar-wrap {
  width: 100vw;
  position: relative;
}

.scroll-box {
  width: 100%;
  height: 100vh;
  overflow-y: scroll;
}

.scroll-box::-webkit-scrollbar {
  width: .4em;
}

.scroll-box::-webkit-scrollbar,
.scroll-box::-webkit-scrollbar-thumb {
  overflow: visible;
  border-radius: 4px;
}

.scroll-box::-webkit-scrollbar-thumb {
  background: rgba(0, 0, 0, .2);
}

.cover-bar {
  position: absolute;
  background: #fff;
  pointer-events: none;
  height: 100%;
  top: 0;
  right: 0;
  width: .4em;
  -webkit-transition: all .5s;
  opacity: 1;
}

.cover-bar.hidden {
  opacity: 0;
}

JavaScript

document.querySelector('.scroll-box').addEventListener('scroll', hideCoverBar);
document.querySelector('.scroll-box').addEventListener('mousemove', hideCoverBar);
var showTimeout;
function hideCoverBar() {
  document.querySelector('.cover-bar').classList.add('hidden');
  clearTimeout(showTimeout);
  showTimeout = setTimeout(showCoverBar, 1000);

}
function showCoverBar() {
  document.querySelector('.cover-bar').classList.remove('hidden');
}