Making unordered list scrollable

Question

I have an unordered list which is inside a div tag and initially the list is empty. When items are added on to it, it will expand and I want it to be scroll-able once it the length of the list exceeds that of the web page, i.e I do not want the web page to scroll, I want only the unordered list to scroll. However currently, the scroll bar for my unordered list is not appearing.

My html code is:

<div style="width: 25%; float: right; " class="online_users">
  <div class="panel panel-primary">
    <div class="panel-heading">
      <span class="glyphicon glyphicon-comment"></span> Online Users
        <ul id="ListOfOnlineUsers" style="overflow: auto;height:100%; word-wrap: break-word;" class="list-group">  
        </ul>
    </div>
  </div>
</div>

My CSS code is

html{
    height: 100%
}

{
  margin: 0;
  padding: 0;
}
body {
  padding: 50px;
  font: 14px "Lucida Grande", Helvetica, Arial, sans-serif;
  height: 100%;

}

a {
  color: #00B7FF;
}

.chat
{
    list-style: none;
    margin: 0;
    padding: 0;
}

.chat li
{
    margin-bottom: 10px;
    padding-bottom: 5px;
    border-bottom: 1px dotted #B3A9A9;
}

.chat li.left .chat-body
{
    /*margin-left: 60px;*/
}

.chat li.right .chat-body
{
    /*()margin-right: 60px;*/
}


.chat li .chat-body p
{
    margin: 0;
    color: #777777;
    word-wrap: break-word;
}

.panel .slidedown .glyphicon, .chat .glyphicon
{
    /*margin-right: 5px;*/
}

.panel-body
{
    overflow-y: scroll;
    top: 1em;
    left: 1em;
    height: 250px;
}

::-webkit-scrollbar-track
{
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
    background-color: #F5F5F5;
}

::-webkit-scrollbar
{
    width: 12px;
    background-color: #F5F5F5;
}

::-webkit-scrollbar-thumb
{
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,.3);
    background-color: #555;
}


/* CSS used here will be applied after bootstrap.css */
body, html {
    height: 100%;
    background-repeat: no-repeat;
    background-image: linear-gradient(rgb(154, 145, 162), rgb(12, 97, 33));
}

.card-container.card {
    width: 350px;
    padding: 40px 40px;
}
.card {
    background-color: #F7F7F7;
    /* just in case there no content*/
    padding: 20px 25px 30px;
    margin: 0 auto 25px;
    margin-top: 50px;
    /* shadows and rounded borders */
    -moz-border-radius: 2px;
    -webkit-border-radius: 2px;
    border-radius: 2px;
    -moz-box-shadow: 0px 2px 2px rgba(0, 0, 0, 0.3);
    -webkit-box-shadow: 0px 2px 2px rgba(0, 0, 0, 0.3);
    box-shadow: 0px 2px 2px rgba(0, 0, 0, 0.3);
}
.btn {
    font-weight: 700;
    height: 36px;
    -moz-user-select: none;
    -webkit-user-select: none;
    user-select: none;
    cursor: default;
}

.form-signin input[type=text],
.form-signin button {
    width: 100%;
    display: block;
    margin-bottom: 10px;
    z-index: 1;
    position: relative;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
}

.btn.btn-signin:hover,
.btn.btn-signin:active,
.btn.btn-signin:focus {
    background-color: rgb(12, 97, 33);
}

Answer 1

Set the list's max-height to the height of its parent (or whatever you need it to be) and then set its overflow property to auto.

Here's a quick example:

div{
  border:1px solid #000;
  font-family:arial;
  height:100px;
  width:200px;
}
ul{
  list-style:none;
  max-height:100px;
  margin:0;
  overflow:auto;
  padding:0;
  text-indent:10px;
}
li{
  line-height:25px;
}
li:nth-child(even){
  background:#ccc;
}

<div>
  <ul>
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
  </ul>
</div>