JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
    <div id="sidebar"></div>
    <div id="main">
        <div id="content">
            <p id="stuff">blahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblah</p>            
        </div>
        <div id="pager">
            <div id="item-list">
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
                <div class="item"></div>
            </div>
        </div>
    </div>
</div>

CSS

#wrapper {
  display: flex;
  width: 100%;
  height: 100%;
}

#sidebar {
  overflow: auto;
  flex: 0.25;
  border:3px solid green;
  min-height: 200px; 
}

#main {
  display: flex;
  flex: .75;
  flex-direction: column;
  border:3px solid orange;
}

#content {
  position: relative;
  width: 100%;
  flex: 0.85;
  border: 3px solid blue;
}

#pager {
  display: flex;
  position: relative;
  width: 100%;
  background-color: #fff;
  border: 3px solid pink;
  flex: 0.15;
}

#item-list {
  border: 1px solid black;
  width: 100%;
  overflow-x: scroll;
  overflow-y: hidden;
  white-space: nowrap;
}

.item {
    display: inline-block;
    padding: 5px;
    width: 200px;
    height: 200px;
    background-color: red;
}

#stuff {
    height: 200px;
}

*{
    margin: 3px;
}