Steps to reproduce:
- Enable 2017 wikitext editor beta feature.
- Go to some page that allows using the new section button (most talk pages, and some other pages), e.g. https://en.wikipedia.org/wiki/User_talk:Deskana/sandbox.
- Click the + button in the toolbar to add a new section.
A grey bar is displayed across the screen. It doesn't impair the functionality at all, but it clearly shouldn't be there.