overflow (scroll) - 100% container height

#container {
    width: 500px;
    border: 3px solid red;
    margin: 0 auto;
    position: relative;
}

#sidebar {
    position: absolute;
    left: 0;
    top: 0;
    width: 150px;
    height: 100%;
    background-color: yellow;
    overflow-y: scroll;
}

#main {
    margin-left: 150px;
}

p, ul {
    padding: 10px;
}

<div id="container">
    <div id="sidebar">
        <ul>
            <li> line 1 </li>
            <li> line 2 </li>
            <li> line 3 </li>
            <li> line 4 </li>
            <li> line 5 </li>
            <li> line 6 </li>
            <li> line 7 </li>
            <li> line 8 </li>
            <li> line 9 </li>
            <li> line 10 </li>
            <li> line 11 </li>
            <li> line 12 </li>
            <li> line 13 </li>
            <li> line 14 </li>
            <li> line 15 </li>
            <li> line 16 </li>
            <li> line 17 </li>
            <li> line 18 </li>
            <li> line 19 </li>
            <li> line 20 </li>
        </ul>
    </div>
    <div id="main">
        <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. </p>
    </div>
</div>