One drawback of details was that cmd+f search wouldn't play nicely when the details was closed. But now there's a hidden="until-found" you can put on child content, along with an associated event. So you can open the details when a user searches :) super useful
You don't need the hidden="until-found" for details/summary, because that has those semantics automatically, but you can use that for other elements that behave similarly (for example tabs, which can't quite correctly be implemented with details/summary, and so needs to be done by hand).
Also I think the event isn't currently emitted consistently on all browsers (and maybe not at all for hidden="until-found"?) so unfortunately you can't quite rely on that yet if you need to sink some JavaScript state to your html. But in general, yeah, this is a really cool feature.
Yeah, support was patchy until recently (and I think that behaviour might not even have been standardised?) so I think a lot of people have assumed that if you want that functionality you need to do something extra.
Yes, Google started revealing the contents of <details> a few years ago, long after the element was supported in all browsers. Firefox added support earlier this year and Safari just added it.
Supporting the behavior was related to changing the user agent CSS when they're closed and the other browsers implemented it and hidden=until-found at the same time.
