I think the borderless design is significantly less intuitive:
The title appears to be part of the post body editor. Fine. But if I type the title and then press the Enter key, the cursor doesn’t move to the next line. It feels like I’m just stuck on the first line of the post without any way to progress. I don’t think the user will intuitively understand they must click on the next line or press Tab to continue. If you want to make title input part of the post body editor, then it must work just like any other line in the editor. If it is going to continue to be treated as a separate input field, then go back to the old design where that nature was visually communicated to the user.
The lack of a border between the post editor and the preview (when the composer is in markdown editor mode) is too confusing. These are two separate panels. That fact should be visually communicated to the user by a border between the two.