JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://cdn.bootcss.com/bootstrap/3.2.0/css/bootstrap.min.css">
<div class="main">
<div class="col-left">
    寬200，<br/>
    左邊有多長右邊就有多長
    <ul>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
      <li>test</li>
    </ul>
  </div>
  <div class="col-right">
      <div>test</div>
      <div>test</div>
      <div>test</div>
    <!-- Bootstrap -->
    <div class="panel panel-primary">
      <div class="panel-heading">
        <h3 class="panel-title">Panel title</h3>
      </div>
      <div class="panel-body">
        Panel content
      </div>
    </div>
    <!-- Bootstrap -->
  </div>
    </div>

CSS

.main {
    position:relative;
}
.col-left {
  width: 200px;
  background: #f6f6f6;
}
.col-right {
    position:absolute;
    left:200px;
    right:0px;
    top:0px;
    bottom:0px;
  /*margin-left: 200px;*/
}
.col-right::before {
    display:block;
    height:100%;
    width:0px;
    float:left;
    content: " "
}