This was way more than a week ago, I remember installing an extension for exactly this months ago. Then it came back, then it disappeared again [...] - this might be betatesting though.

I can’t speak authoritatively about this specific instance, but I use multiple unlinked google products across multiple devices and they absolutely split test UI changes. It can be fairly nefarious on the advertiser/publisher side of things if you actually understand what their attempting to do.

It’s really too bad, because Google traded their pristine reputation for possibly a slight boost in their near term earnings.

You have to make the window larger (than "mobile-sized") to see it.