Every time I visited the API browser of Prismic.io I ran into an issue where the top region is not showing at all. Inspection deems that part as "display: none" by an "injected stylesheet". After digging into it, those are styles injected by chrome browser extensions making the element quite passive. Redefining its styles got me nowhere, still have not figured out why injected stylesheets override page styles in such an aggressive manner.
The culprit was Ad Blocker, which is very useful, but unfortunately, it considered .banner style as an Ad. The solution was to deactivate the extension on that page.