Steps to replicate the issue (include links if applicable):

• Ensure the 2017 wikitext editor is enabled
• Start a new topic on a discussion page by using "Add topic" (or just click here)

What happens?:
The "New section" placeholder overlaps with line numbers

What should have happened instead?:
The placeholder should not overlap

Other information (browser name/version, screenshots, etc.):

This was reported on itwiki's Village pump (technical). As far as I know, CodeMirror preferences are not available on the 2017 wikitext editor, so line numbers cannot be disabled by the user.