I generally use the Project tool window in the "Packages" view. I've got my tests and production code in different directories and the Packages view lets me see code and associated tests side-by-side. I was hoping to use the Navigation Bar to browse my project this way, but the Navigation Bar doesn't have an option to show collapsed packages. I realize this may not be the intent behind the Navigation Bar, but the package view is useful when navigating my project, so I think it could be a good fit.