I'd never used Fastmail and as a sometimes-UI guy, thought I'd check out their 30 day trial to see what all the praise was about. I saw your comment before going, so kept that in mind.
With a brand new user account, I was not at all able to reproduce your issue. I closed the browser and cleared cache and all that, but I did not see an entire page refresh at any point.
Perhaps there's some weird caching issue with your browser, or maybe you've got a browser extension misbehaving? Maybe I'm just lucky? Idk, but hopefully this helps.
