How can I get emails to open in a separate window in Mac Mail (Fullscreen)

Every time I go to reply to an email or double click to open it when mail is in full screen, the email opens within Mail, meaning whilst i'm writing an email, I can't check back on other emails or have multiple drafts open etc. Is there a way of solving this and getting emails to open in a separate window even when mail is in fullscreen mode? I can't seem to find anything anywhere!!

Thanks!

PS sorry for putting this in the obviously wrong community - but there isn't one for mail!