how to make this submenu within the menus scrollable so that they don't go off screen
my html
<nav>
    <ul>
        <li><a href="#">parent</a></li>
        <li><a href="#">parent</a></li>
        <li>
            <a href="#">parent width child</a>
            <ul>
                <li><a href="#">child</a></li>
                <li><a href="#">child</a></li>
                <li>
                    <a href="#">child with children</a>
                    <ul>
                        <li><a href="#">grand child</a></li>
                        <li><a href="#">grand child</a></li>
                    </ul>
                </li>
            </ul>
        </li>
        <li><a href="#">parent</a></li>
    </ul>
</nav>
my css
/* reset padding and margin where necessary etc. */
ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
}
a {
    text-decoration: none;
}
/* just some quick demo styles for color whatnot */
nav {
    background: black;
    color: white;
}
nav ul ul {
    background: #555;
}
nav ul ul ul {
    background: #999;
}
nav a  {
    color: white;
    white-space:nowrap;
}
nav a:hover  {
    background: #f80;
}
/* important functional styles */
nav > ul:after {
    /* clear the float */
    content:'';
    clear:both;
    display: block;
}
nav li  {
    /* for the topmost level we want them to float.  will be overridden */
    float:left;
}
nav li a {
    /* always apply padding and display block to the a.  better user experience. */
    display:block;
    padding: 10px;
}
nav li ul li {
    /* overridden floating here */
    float: none;
}
/* here is where all the positioning takes place */
nav li {
    position:relative;
}
nav li ul {
    position:absolute;
    left: 0; /* for top most level lets align to the left */
    top: 100%; /* and have it at the bottom of the parent */
    display:none; /* hide initialy */
}
nav li ul li ul {
    left: 100%; /* for grandchild level lets align to the right of the list item */
    top: 0; /* and have it at the top of the parent li */
}
nav ul li a:hover + ul,
nav ul li a + ul:hover {
    /* show only if the element or the immediately preceding anchor is hovered*/
    display:block;
}
/* enjoy! */
plz see the jsfiddle
when there are more than one submenu itemor like 50 such items then how to make it scrollable?
 
     
    