nav会浮动到中间位置

<nav class="mui-bar mui-bar-tab j-tool-bar">
<a class="mui-tab-item" href="#" id="emailButton">
<span class="mui-icon mui-icon-email"></span>
<span class="mui-tab-label">邮件</span>
</a>
<a class="mui-tab-item" href="#" id="followUpButton">
<span class="mui-icon followUpBut"></span>
<span class="mui-tab-label">跟进</span>
</a>
<a class="mui-tab-item" href="#" id="saveButton">
<span class="mui-icon mui-icon-checkmarkempty"></span>
<span class="mui-tab-label">保存</span>
</a>
<a class="mui-tab-item" href="#customSettingPopover" id="moreButton">
<span class="mui-icon mui-icon-more"></span>
<span class="mui-tab-label">更多</span>
</a>
</nav>
点击一个输入框，弹出输入法后，滑动页面，底部的nav会上浮。
详情看附件。