fix some nasty combobox issues

this pr fixes nasty issues after clear text button enhancement

![image](https://f.cloud.github.com/assets/658431/1394818/4fb5986c-3c2f-11e3-9ab7-c0d014ee61cf.png)
![image](https://f.cloud.github.com/assets/658431/1394820/5198daea-3c2f-11e3-9b86-c2b6e99737f0.png)
![image](https://f.cloud.github.com/assets/658431/1394821/53c96532-3c2f-11e3-90f8-0605ece060cf.png)

after pr

![image](https://f.cloud.github.com/assets/658431/1394824/5d760acc-3c2f-11e3-8c60-8938eec41e82.png)
![image](https://f.cloud.github.com/assets/658431/1394827/5f4bc152-3c2f-11e3-8940-8c5777c19772.png)