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="col-right">
    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-left">
    <!-- 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>

CSS

.col-right {
  float: left;
  width: 200px;
  background: #f6f6f6;
}
.col-left {
  position:relative;
  margin-left: 200px;
}
.col-left .panel-body:after {
    clear:none;
}