I'm interested, both as a Fastmail customer, and a software developer: what does this let you do that a PWA doesn't? Perhaps not become the default email client? Is there anything else?
The main thing is better integration with the OS. So no browser chrome (even as a PWA, the browser adds buttons or a toolbar over the top of the app), integration with the Mac menu bar, native context menus, the OS semi-transparent background for the frame so it feels like it belongs.
