Handle the queue menu not fitting vertically (bug 985996)

https://bugzilla.mozilla.org/show_bug.cgi?id=985996

This might not be the best fix but it's better than what we've got now.

Before
![before mp4](https://cloud.githubusercontent.com/assets/211578/3514380/bbb317a8-06cb-11e4-9c2f-e58c56d8231c.gif)

After
![after mp4](https://cloud.githubusercontent.com/assets/211578/3514388/c2fa4842-06cb-11e4-8385-fbe6254a9aa1.gif)

why this number? and we're using pixels below. why using rems here?