To display the standard tools on a panel, the 'title' property should be set.
<div id="additional-info" class="easyui-drawer" data-options="title:' ',closable:true,collapsible:true,width:350">
...
</div>
You can also custom a collapsible button on a drawer.
<div id="additional-info" class="easyui-drawer" data-options="width:350">
<button class="easyui-linkbutton" iconCls="icon-no" plain="true" onclick="$('#additional-info').drawer('collapse')"></button>
<p style="text-align:center;margin:50px 0;font-size:16px">Content...</p>
</div>