I have a HTML and CSS files containing the following code:

<div >
   <div ></div>
   <div >
       <div >
           <h1>Item</h1>
           <h1>Item</h1>
           <h1>Item</h1>
                ..
       </div>
       <div ></div>
   </div>
</div>

html, body{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body{
    width: 100vw;
    height: 100vh;
}

.app{
    display: flex;
    flex-direction: column;
    width: 100%;
    height: 100%;
}

.header{
    width: 100%;
    min-height: 50px;
    background-color: rgb(255, 235, 147);
}

.main{
    display: flex;
    width: 100%;
    height: 100%;
}

.container_1{
    display: flex;
    flex-direction: column;
    width: 200px;
    height: 100%;
    background-color: rgb(255, 147, 147);
    overflow-y: auto;
}

.container_2{
    width: 200px;
    height: 100%;
    background-color: rgb(147, 147, 255);
}

Which looks like this: