This used to work around last year - then now I noticed the nutshell links I had put into details elements were not opening anymore.
This stands true in the interactive demo as well.

I've encountered the same issue on Firefox and Chrome both, and both in the demo box and regular blank html pages.
I'm sorry I can't give any more valuable information, I'm not knowledgeable in this. All I can gather from the inspect tab is that, when a nutshell link gets placed inside a details element, it simply keeps being treated as a regular simple link, and no changes are applied to it whatsoever.
This used to work around last year - then now I noticed the nutshell links I had put into details elements were not opening anymore.
This stands true in the interactive demo as well.

I've encountered the same issue on Firefox and Chrome both, and both in the demo box and regular blank html pages.
I'm sorry I can't give any more valuable information, I'm not knowledgeable in this. All I can gather from the inspect tab is that, when a nutshell link gets placed inside a details element, it simply keeps being treated as a regular simple link, and no changes are applied to it whatsoever.