On the Comments tab add/edit functionality, clicking Upload & delete, Preview, or Cancel button always submits the form and saves the contents.

This issue can be replicated reliably using (3.6dev+ (Build: 20181031). Please see steps below:
1. Create a course.
2. Turn editing on.
3. Add a Wiki activity and create your first page and save (creole or nwiki format).
4. Go to the Wiki activity, and click the "Comments" tab.
5. Click the "Add comment" link.
6. Type a content in the text-area.
7. Attach a file in the "Attachments" area.
8. Click "Upload & Delete".
Result: The form will be submitted and the contents of the text area are saved.
9. Repeat steps 5, 6 (change the contents of the comment), and 7, then click "Preview".
Result: The form will be submitted and the contents of the text area are saved.
10. Repeat steps 5, 6 (change the contents of the comment), and 7, then click "Cancel".
Result: The form will be submitted and the contents of the text area are saved.