The Fastmail desktop app is literally a wrapped version of their website but with the added feature of... get this... no back button (or equivalent shortcut).

My 30 years of muscle memory using webmail is made useless by this "app" because some web developer somewhere wants to cosplay as a desktop app developer now.

It's not an oversight either. It's an intentional choice to not have a go-back-to-previous-page keyboard shortcut. A customer support person said they would add it as a "feature request". Gee, thanks.

It might be worth learning their hotkeys because they're pretty efficient in many cases (potentially included cases where you want the back button). They work in the browser and the app:

https://www.fastmail.help/hc/en-us/articles/360058753534-Key...

Not saying your complaint is invalid at all.