Yes, “subject keeping” is a good point. And it would also be a requirement for Reply to Message and mailing list support.
So how could the classic subject handling be integrated nicely in the UI?
Idea:
- Non-chat, classic email messages could be displayed with some sort of reply button ↺ .
- Pressing the reply button, could open the text editor box then prefilled with
🔓 Re: The current subject-, and the cursor already positioned behind it.
And concerning your aspect of message threading errors at some contacts’ clients (email client that only sorts by subject text but not based on headers):
As it can not be expected that contacts switch to another email client very quickly: Either a per chat or per contact setting, or even better shipping with a list of affected email clients and automatically using the “subject keeping” if a recipient uses such an email client.