At phab:T247485#6086554, @Iamjessklein commented
One additional thing that felt off was what happens when a user doesn't add a title to their discussion but posts it. If a user doesn't provide a subject on their new topic post, the post goes under the previous topic. I find that to be unexpected and something that we should not reproduce when we do our iteration.
And at phab:T247485#6067031
Many participants only left a topic and were not directed how to write the initial comment.
I'm not sure if that means they filled in the “Subject/headline” box but added no text in the main editor box?
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Try doing the same thing on Mobile web: the Publish button is disbled if either box is blank. (Fields are labelled "Subject" and "What is on your mind?" in mobile.) Fix: apply the same logic to Desktop web.