应用overflow：auto时，按箭头显示消失

Question

我正在尝试设置select下拉菜单的样式，以便在到达150px高度时溢出。问题是，当我应用overflow: auto属性时，包含指向选择的箭头的伪后元素会消失。 这里我们有代码：

.dropdown-menu{
    background-color:#f2f2f1;
    min-width: 100px;
    border:none;
    border-top:5px solid #b4b4b4;
    border-radius:0;

    >li>a{
        font-size: 14px !important;
        color:#818181 !important;
        &:hover, &:active{
            color:#003e59 !important;
        }
    } 
    .divider{
        height:2px;
        margin: 2px 0;
        background-color:#cdcccd; 
    }
    &:after {
        bottom: 100%;
        left: 25%;
        border: solid transparent;
        content: " ";
        height: 0;
        width: 0;
        position: absolute;
        pointer-events: none;
        border-color: rgba(136, 183, 213, 0);
        border-bottom-color: #b4b4b4;
        border-width: 12px;
        margin-left: -12px;
    }
}

 <div class="input-group">
                                                <label>Jurisdiction</label>
                                                <button class="btn btn-default btn-sm dropdown-toggle" type="button" data-toggle="dropdown">
                                                    Select Jurisdiction <span class="spacer"></span><span class="caret"></span>
                                                </button>
                                                <ul class="dropdown-menu" role="menu">
                                                    <li><a href="#">Jurisdiction 1</a></li>
                                                    <li class="divider"></li>
                                                    <li><a href="#">Jurisdiction 2</a></li>
                                                    <li class="divider"></li>
                                                    <li><a href="#">Jurisdiction 3</a></li>
                                                </ul>
                                            </div>