css - 才用fixed定位之后，flex布局的flex-grow属性就挂掉了？

Question

<p class="user_section">

    <ul>
        <li><a href="#" title=""><img src="../images/user_section/home.jpg" width="21" alt=""></a></li>
        <li><a href="#" title=""><img src="../images/user_section/share.jpg" width="21" alt=""></a></li>
        <li><a href="#" title=""><img src="../images/user_section/main.jpg" width="42" alt=""></a></li>
        <li><a href="#" title=""><img src="../images/user_section/cat.jpg" width="24" alt=""></a></li>
        <li><a href="#" title=""><img src="../images/user_section/mine.png" width="23" alt=""></a></li>
    </ul>
</p>



.user_section{ position: fixed;}
.user_section ul{ height: 49px; box-sizing: border-box; display: flex; flex-direction:row; justify-content:space-between; align-items:center;  background: white; border-top: 1px solid #e6e6e6; }
.user_section li{ flex-grow:1}