Agree that it is useless to try to respond to specifics of an email if one cannot either pop out the email that is being replied to for reference, or pop out the draft email as someone else suggested.

I imagine that the current Proton Mail app is a very simple "copy" of the Web app that is used when logging in directly to Proton. A complete rewrite is probably in order to come up with a "modern" web application. (I have to shake my head at the constant blue banner asking to make the Proton app the default app.)

Fastmail seems to have been working through a good app, albeit can use polish here and there, but they seem to be updating regularly.

I understand the challenge, but if you have the resources, this (along with other "modern" features) should strongly be considered.

it is vital that i can do this

This is a complete failure in workflow and makes using the app so much slower in any usecase which doesn't involve simply replying to a quick email.

Two biggest issues:

a) if you reply to an email, the composer covers the text of the original mail so you can't read it
b) if you open an attachment it completely covers the whole window so you can't see anything else.

This functionality has been standard on all other mail clients I've used for a very long time and seems like a very odd thing to overlook.

Proton devs, take note! Separating a new message composition into to an independent draggable window is essential for business use and is pretty much an expected feature by anyone using a multi-monitor desktop display. This feature is standard on MS Outlook.