The really fun part, though: the summary content isn't just a menu icon, it's a trigger for some (currently unpublished) Turbolinks variant. The menu content is loaded (and subsequently 304'd) dynamically as a server-rendered partial (sans layout).
Most tellingly, if you visit the menu content's XHR URL with regular browsing then hey.com renders that menu as the sole content in a page layout.
We can't see the server-side parts yet, but if that becomes an out-of-the-box end-to-end behaviour within Rails, then in conjunction with Stimulus 2 and Action Cable I'm stoked about the possibilities. Especially so since I have a mission to replace all my dropdown & modal boilerplate with <details><summary> and <dialog> elements, and I like to build services that still work if JS is disabled. But it's also good for mundane stuff like progressive form validation in line-of-business apps.