删除SimpleComboBox ext gwt扩展时出现的Shadow

Question

我想删除SimpleComboBox上展开的阴影。 我把阴影作为附图。

下面是我的代码

private SimpleComboBox<String> getSimpleComboBox(List<String> valuesOfComboBox, String objectToDisplay, boolean isVisible, String fieldLabel, boolean isTypeAhead, TriggerAction triggerAction, String id, boolean isShadowVisible){
        SimpleComboBox<String> comboBoxForUserAccount = new SimpleComboBox<String>();
        comboBoxForUserAccount.setVisible(isVisible);
        comboBoxForUserAccount.setShadow(false);

        if(valuesOfComboBox!=null)
            comboBoxForUserAccount.add(valuesOfComboBox);


        if(objectToDisplay!=null)
            comboBoxForUserAccount.setSimpleValue(objectToDisplay);
        comboBoxForUserAccount.setFieldLabel(fieldLabel);
        comboBoxForUserAccount.setTypeAhead(isTypeAhead);
        comboBoxForUserAccount.setTriggerAction(triggerAction);

        if(id!=null)
            comboBoxForUserAccount.setId(id);
        return comboBoxForUserAccount;
    }

感谢