Ah, I see. Thanks for pointing that out. But one needs to know that there is an invisible widget (at least I see nothing at all there, after it has faded out) to hover over. I certainly had no idea.
The idea there is that 'something which has faded out but can be recalled easily' is a fairly common UX/UI design pattern, and so a user will unconsciously know it's there and can easily rediscover it by accident. Plus, everything else on gwern.net makes some sort of use of hover, so it's not like you expect hover to be useless!
Well, OK. Personally, I moved away from using hover for anything important on my sites, because it has such a restricted application. It either doesn’t work or works unpredictably on phones and other touchscreen devices, and is pretty useless for people, like me, who use keyboard navigation. But now I read academic papers using the sioyek PDF reader, which makes such good use of hover that I’m regaining some affection for it. (Hover over a reference to a figure to see the figure pop up without losing your place, for example.)
