This requested feature will depend upon the upcoming Notifications upgrade.
I continue to get caught up in generating a reply to a message without realizing that there are more replies in the topic and that my reply may no longer be applicable. This could include those that I have not yet read or new ones that are arriving while I am composing my reply.
Hence, I would like to see a "New Messages" indicator in the compose window that shows an active count of the messages in this topic that I haven't yet read. That would be best. A second best would be feedback when I hit "Send". It would only come up if I hadn't yet read all replies in the topic.
A tertiary alternative would be making it easier to view the thread easily while composing. As it is now, I have to open up another browser tab and then futz around with drafts which do or don't show up until refreshing the browser, etc. Actually, that might be a worthwhile improvement on its own, even without the new Notification system.