The Proton desktop app combines Mail and Desktop. However, it's one OR the other.

When you need calendar info to write an email, or when you need info from an email to set up a calendar event, this can be pretty painful to use.
The best solution would be to be able to have several windows and/or tabs open. These could be the mail main window, a mail compose window, a single email content, the calendar main window, a single calendar event, or even the preferences and/or settings.

As of now, having a single window feels like being trapped.