textarea summary field not hidden in Olivero

I think you're trying to use Olivero as admin theme. But we don't recommend using Olivero as an admin theme

No I'm not.

Website is the out of the box default on simplytest.me. The standard website theme is Olivero. The admin theme is Claro.

If you use the website theme to edit content, then you run into this problem.

That is not "using it as an admin theme", it's editing content.

And in any case that is irrelevant, it is still a bug somewhere.

I would say it's a bug of Olivero theme.
This functionality (regarding hide/show summary) comes from text.js from core module text

This script is loaded on the page, but doing nothing with Olivero theme, because it's expecting to have .js-text-format-wrapper selector in DOM and in Olivero this classname doesn't exist. In Claro/stable9/starterkit/umami this classname exist (no matter how exactly this classname is added: by using twig override of text-format-wrapper.html.twig or by hacking preprocess hooks).

Another strong reason why this is a bug - is that user CAN have access to form display pages.
This is a quick-fix which i'm sure will be enough (maybe even with missing styles for button html tag for Hide/Show summary text).

Also screens before patch:

And after patch:

Seems like something we could add a simple assertion for.

If it is being left to the themes to add the class, but the core code expects it to be there, is that an issue with the core Text module, or the textarea field widget?