Fix bubble padding.

fix #3823

### Before

![fix-bubble-pugging-before](https://f.cloud.github.com/assets/72138/500798/3b381388-bc82-11e2-9c82-e4a6a3803189.png)

### After

![fix-bubble-pugging-after](https://f.cloud.github.com/assets/72138/500799/43163d5a-bc82-11e2-8eb3-d9eed6aa1dc5.png)