It's a zindex issue on the header

No, that's not the issue. I intentionally scrolled down before making the screenshot to showcase that tooltip falls out of viewport. Bootstrap tooltips/popovers don't have this issue.

Whatever is the cause it used to work correctly. it is such an obvious issue it would never habe been merged like this. So there is a change somewhere since this was merged

No, it was always like this. These tooltips can't position themselves based on available space. Not sure if this can be done with CSS alone. I just want to point out that we'll start running into this issue more often if the intention is to replace BS tooltips with these.

Also, a more specific selector should be used. Have you seen #25161 (comment)?

As can be seen at the time it was committed in alpha 10 it happily sat on top of the header which it will not do now as it goes behind the header which is a bigger issue that it not being in the viewport

Brian, try scrolling down a little bit more so that the search input is inline with the top of the viewport.
If I remember the code for this, it's just basic CSS. There's no JS involved, so it won't change direction based on available space. BS tooltips (popperJs) do this but I don't think core uses them anymore

yes it is just basic css - on purpose

Personally I dont care about the viewport - you can always scroll

What I do care about though is that a recent change has resulted in the info being hidden permanently

Without actually firing up the page, I can't actually see any z-index issues based on looking at the SCSS. The tooltip has a higher z-indez than the header.

While I wrote z-index that might not be the problem but it was easy to use as a description of the problem

Resolved with #28258

That is completely unrelated. This issue is not resolved.