Steps to reproduce:

1. Add a new section to a page, e.g., by clicking on https://en.wikipedia.org/wiki/User:Whatamidoing_(WMF)/sandbox?veaction=editsource&section=new in Safari
2. Take a moment to admire the fact that the cursor is already in the right place.
3. Type the subject heading.
4. Press tab when you're ready to type the body of your message, because you've done this 30,000 times before, so you're not paying attention.
5. Hey, what's the cursor doing in the URL bar?
6. File bug report.

Problem exists in Safari 10 on macOS Sierra 10.12.2 but not in Firefox 50.