improve transitions for hovercat drop-down menu at desktop width (bug 1096654)

https://bugzilla.mozilla.org/show_bug.cgi?id=1096654

![screencast](https://cloud.githubusercontent.com/assets/203725/4986541/3891af82-693c-11e4-86f5-56eeb5aa06b7.gif)

Is the doubledash something we're introducing?