bootstrap 的模态框中根据滚动高度距离冻结（固定）表头

html部分

<div class="modal inmodal" id="alarmModal" tabindex="-1" role="dialog" aria-hidden="true">
    <div class="modal-dialog" style="width: 80%">
        <div class="modal-content animated bounceInRight">
            <div class="modal-header">
                <div  class="close" data-dismiss="modal">
                    <img src="/static/manager/img/new/close.png" alt="">
                </div>
                <h4 class="modal-title">详情</h4>
            </div>
            <div class="modal-body" style="height: 400px;overflow: auto;">
                <table class="table table-bordered historyInfoList" id="mytable" style="display: none">
                    <thead>
                    <tr>
                        <th class="col-sm-1">通电电位（V）</th>
                        <th class="col-sm-1">断电电位（V）</th>
                        <th class="col-sm-1">交流电压（V）</th>
                        <th class="col-sm-1">直流电压（V）</th>
                        <th class="col-sm-1">直流电流（mA）</th>
                        <th class="col-sm-1">自然电位（V）</th>
                        <th class="col-sm-1">电池电压（V）</th>
                        <th class="col-sm-2">上报时间</th>
                    </tr>
                    </thead>
                </table>
                <table class="table table-bordered historyInfoList">
                    <thead id="datatable">
                    <tr>
                        <th class="col-sm-1">通电电位（V）</th>
                        <th class="col-sm-1">断电电位（V）</th>
                        <th class="col-sm-1">交流电压（V）</th>
                        <th class="col-sm-1">直流电压（V）</th>
                        <th class="col-sm-1">直流电流（mA）</th>
                        <th class="col-sm-1">自然电位（V）</th>
                        <th class="col-sm-1">电池电压（V）</th>
                        <th class="col-sm-2">上报时间</th>
                    </tr>
                    </thead>
                    <tbody>
                    </tbody>
                </table>



            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-white" data-dismiss="modal">关闭</button>
            </div>
        </div>
    </div>
</div>

注：重点在表格部分

jq部分：

<script type="text/javascript">

    $(function(){

        //根据滚动窗体和父窗体top距离进行判断是否冻结表头
        $(".modal-body").scroll(function(event){

            let top = $("#datatable").position().top;
            if(top<=0){
                $("#mytable").css({"display": "","position": "fixed","top":"65px","width": "calc(100% - 65px)"});
            }else{
                $("#mytable").css({"display": "none"});
            }
        });
            });
</script>