When opening a second session of the 'new experience' from the mailings/templates/data areas of the tool (by right-click>open link in new tab/window), the the top-nav and ribbon-nav don't display in second session.

In the classic experience the ability to right-click and open new tab/window with full navigation functionality helped create efficiencies in workflow (it offered a convenient way to navigate to and check other mailings, templates, data jobs etc without having to navigate away from or save progress in the existing session).

(The attached screenshot shows the missing top-nav and ribbon-nav)

UPDATE 16 Jan 2019:

The issues our team were experiencing when opening multiple browser tabs or windows appear to have been fixed - full navigation is now present and working when additional sessions are opened.