Per https://meta.trac.wordpress.org/ticket/6168 some codex pages have a cached header HTML + CSS which are incompatible with one another, causing a display mismatch.
While we’re migrating away from the Codex, it appears likely to remain for some time.
Can we please
1. Flush the codex cache
2. Lower the cache TTL, perhaps to a week or two, to avoid the need for future system requests?