Cannot set a reviewer when editing a pull request

When editing a pull request the reviewer user picker does not show. The REST API call to look up the username succeeds but display fails to render content.

User reports this occurred upon upgrade from 2.7 to 2.8.

I'm using firefox 24.0 on mac os x 10.8.4. I tried in safari too, and I tried clearing caches.

My test confirms that this was working in 2.7.0, tested with Chrome Version 29.0.1547.76 m

Note: Reviewers can be added when creating the pull request.

There is an issue with the z-index of the dropdown which means that it is being hidden behind other page elements.

This also affects adding a branch permission, @ mentions and the participants dropdown in a pull request.