It first I would also have expected vh and vw to do this, i.e. do what dvh and dvw now do. And I think on purely theoretical grounds that would be the correct b

It first I would also have expected vh and vw to do this, i.e. do what dvh and dvw now do. And I think on purely theoretical grounds that would be the correct behavior, after all the view port dimensions do actually change. Well, unless you think of those dynamic elements as overlays in front of the view port. But what would be the implications? Every time some of those dynamic elements appear, everything based on the view port dimensions would also change in size. There are certainly cases where this is exactly what you want but I guess more commonly you would not want to have things change. There is a real difference between changing the size of your browser window where you want things to adapt and temporarily showing some additional user interface where you do not want everything to move around immediately. This seems actually to suggest that those dynamic elements should be overlays and not change the view port size, but I guess only if one focuses on this single issue. If done as overlays, then you could never see the very top or bottom of a page while those dynamic elements are visible which is not good eitehr.