[acdha]

In the screenshots shown, most of the apps do not have menubars which extend to that space at that display resolution — this could definitely be more of an issue for people who increase the size for accessibility reasons.

From the full-screen screenshots, it appears that it's simply black across the top of the screen unless the application has been designed to support it. That looked like the regular Safari developer tools with an unbroken bar slightly lower down in the video.