list-group bootstrap scroll not working as expected

I have a list-group inside a panel, but i need to use an horizontal scroll because list values are very long. I did this using css and overflow-y property, but when i move the scroll, the list group lines are missing. This is my code:

HTML:

<div class="panel panel-primary">
    <!-- Default panel contents -->
    <div class="panel-heading">Visitantes</div>

    <!-- List group --> 
    <ul class="list-group s-scroll select-cursor">
        <li class="list-group-item" ng-repeat="v in visitors" ng-click="showVisitor($index)" 
        ng-class="{selecteditem: $index == selected2}">{{v.nombre}}</li>
    </ul>
</div>

CSS:

.s-scroll {
    max-height: 200px;
    overflow-y:auto;

}

.select-cursor {
    cursor: pointer;
}

.selecteditem {
    background-color: activecaption;
    color:white;
}

Picture:

Any idea?? Thanks in advance!!