SOURCE

<div class="box">
	<div class="left"></div>
	<div class="right">
		<div class="content">
			sfffeeeerfffff
		</div>

		<div class="content">
			sfffeeeerfffff
		</div>
		<div class="content">
			sfffeeeerfffff
		</div>
		<div class="content">
			sfffeeeerfffff
		</div>

	</div>
</div>
</div>

.box {
    display: flex;
    height: 100px;
    overflow: auto;
}

.left {
    width: 100px;
    background: green;
}

.right {
    display: flex;
    flex: 1;
    overflow: auto;
    background: #f0f;
}


.content {
    margin-right: 5px;
    display: inline-block;
    width: 300px;
    height: 100%;
    background: red;
    flex-shrink: 0;
}