What is this?

A new feature? That's one dynamic search bar.

@SolidSnake35: Is that happening to you regularly? Or just a one-time thing?

Quite often. Usually when I scroll down a long way and come back up.

@SolidSnake35: What's your OS/browser version?

Windows 7, Firefox 47 (updating as I type).

Seems it's the whole top banner; not just the search bar.

Still happening...

@SolidSnake35: We've been looking at this, but we can't replicate it, and it just doesn't look like something that could even be caused by code. It looks like the browser is choking somehow, perhaps because the system is low on resources? Hard to say. Do you see it in any other browsers?