#menu {
    width: 150px;
    background: #000;
}

#menu ul {
    list-style: none;
    padding: 0;
    margin: 0;
}

#menu ul li {
    position: relative;
    border-bottom: 1px solid #fff;
}

#menu ul li a {
    display: block;
    color: #fff;
    padding: 10px;
    text-decoration: none;
}


#menu ul li a:hover {
    background: darkgray;
}


#menu ul ul {
    display: none;
    position: absolute;
    left: 150px;
    top: 0;
    width: max-content;
    background: #000;
}

#menu ul li:hover > ul {
    display: block;
}