The width is now defined on the entire list instead of the inner elements, similarly to other Photon panels like the main menu. This is a prerequisite for preventing layout issues while removing the "download" binding.
Differential Revision: https://phabricator.services.mozilla.com/D6284
This is part of the work to remove XUL overlays. The overlay was originally
created to help startup performance, but using a hidden panel instead seems to
have no detrimental effects to performance and is more straightforward.
MozReview-Commit-ID: JlWcZEhPXyH
