As an instructor or student, When starting a new discussion in the forum, a teacher cannot click on "post to forum" without a message. No error message or warning, it just doesn't highlight the button which is the expected behavior . It would be better if the user is prompted that the message cannot be blank

This can be reproduced in ios in Moodle generic app and in customized moodle app
In Android or in browser , the message box highlights in red, making it intuitive to the user to enter a message and then post the form