当下拉列表打开时，angularjs popover与ui-select重叠

Question

当angularjs ui-select下拉打开时，popover会相互重叠，如屏幕截图所示。

 <div class="form-group required">
    <a href="" class="fa icon-info" popover-placement="left" popover="{{'dfgf_popover'|lang}}" ></a>
    <ui-select required ng-model="country.selected" theme="selectize" ng-disabled="disabled" style="width: 300px;">
       <ui-select-match placeholder="Select or search a country in the list...">
          <span>{{$select.selected.name}}</span>
          <button class="clear" ng-click="clear($event)">X</button>
       </ui-select-match>
       <ui-select-choices repeat="country in countries | filter: $select.search">
          <span ng-bind-html="country.name | highlight: $select.search"></span>
          <small ng-bind-html="country.code | highlight: $select.search"></small>
       </ui-select-choices>
    </ui-select>
</div>

如何编写css代码来解决此问题。