SOURCE

<div class="message-box">
  <div class="message-list"></div>
</div>

<div class="person-a">
  <input type="text" class="persona" />
  <button class="senta">发送消息</button>
</div>

<div class="person-a person-b">
  <input type="text" class="personb" />
  <button class="sentb">发送消息</button>
</div>

.message-box{
  width:500px;
  height:500px;
  border:1px solid #ccc;
  margin:20px auto;
  overflow:hidden;
  background:#e2dcdc;
}

.person-a{
  overflow:hidden;
  margin-top:50px;
}