JSFiddle - React, Tailwind, and code Playground

HTML

<select id="select_id" class="select" size="5">
    <option value="1" >test1</option>
    <option value="2" >test2</option>
    <option value="3" SELECTED>test3</option>
    <option value="4" >test4</option>
    <option value="5" >test5</option>
  </select>
  <div id="div_id" class="cover">
  </div>

CSS

.select{
  height:60px;
  position:absolute;
}
 .cover {
 background-color:white;
 position:absolute;
 border-left:solid grey 1px;
 width:16px;
}

JavaScript

$("#div_id").height($("#select_id").height()+2); // the height of the select + 2px to include the border
$("#div_id").css("margin-left", $("#select_id").width()-15); // the width of the select minus the scrollbar