Set popup panel width to container width so we don't get scroll bars.

#### Before
![panel-before](https://cloud.githubusercontent.com/assets/96776/4960546/3c186a12-66c4-11e4-8094-4ddd5d28d71c.png)

#### After
![panel-after](https://cloud.githubusercontent.com/assets/96776/4960547/3c1992e8-66c4-11e4-9211-0273bf64dd20.png)