SOURCE

<div class="box1">
    <div class="box2 active">
        <div class="box3">
            <div class="box4">1</div>
            <div class="box4">2</div>
            <div class="box4">3</div>
            <div class="box4">4</div>
            <div class="box4">5</div>
            <div class="box4">6</div>
            <div class="box4">7</div>
            <div class="box4">5</div>
            <div class="box4">6</div>
            <div class="box4">7</div>
            <div class="box4">5</div>
            <div class="box4">6</div>
            <div class="box4">7</div>
        </div>
    </div>
    <div class="box2">
        <div class="box3">
            <div class="box4">1</div>
            <div class="box4">2</div>
            <div class="box4">3</div>
            <div class="box4">4</div>
            <div class="box4">5</div>
            <div class="box4">6</div>
            <div class="box4">7</div>
            <div class="box4">5</div>
            <div class="box4">6</div>
            <div class="box4">7</div>
            <div class="box4">5</div>
            <div class="box4">6</div>
            <div class="box4">7</div>
        </div>
    </div>
</div>

.box1 {
    display: flex;
    flex-direction: column;
    background: #fff;
    overflow: auto;
}

.box2 {
    display: flex;
}

.box2.active {
    background-color: red;
}

.box3 {
    flex: auto;
    white-space: nowrap;
}

.box4 {
    display: inline-block;
    width: 200px;
    height: 20px;
}