On desktop (Firefox) at least if I navigate to another folder, the URL changes but the browser back button doesn't change the view back to inbox. On mobile (iOS Safari), if I open an email, then try to navigate back, it takes me all the way back to the login page. The app also seems to use old-fashioned #anchor-based URLs rather than the navigation API.

(Hilariously, I found a feedback link but it points to a 404.)