SOURCE

<div class="box">
	<div class="box1"></div>
	<div class="box2">
		<div style="height: 500px;border: 1px solid gold;"> </div>
	</div>
</div>

/* .box {
    height: 300px;
    display: grid;
    grid-template-rows: auto 1fr;
    border: 1px solid red;
}

.box1 {
    height: 100px;
    margin-bottom: 10px;
    background-color: blue;
}

.box2 {
    overflow: scroll;
    background-color: lightblue;
} */

.box {
    height: 300px;
    display: flex;
    flex-direction: column;
    border: 1px solid red;
}

.box1 {
    height: 100px;
    margin-bottom: 10px;
    background-color: blue;
}

.box2 {
 
    overflow: scroll;
    background-color: green;
}