This library is fairly straight forward and perhaps there are a few things we can learn from it.

https://edenspiekermann.github.io/a11y-dialog/
https://github.com/edenspiekermann/a11y-dialog

And perhaps re-confirm that the things we're already doing are actually working right.

One observation from a quick look: When opening a dialog in the OOUI demo at https://doc.wikimedia.org/oojs-ui/master/demos/#dialogs-mediawiki-ltr, the content is not given aria-hidden. There is code for it, and it's setting it on all the script tags in the body (er..), but not on the menu and content layout of the demo. Presumably this is because the dialog window manager is not a child of the body, so it doesn't know to aria-hide its siblings. Perhaps this logic can be improved, or perhaps a root node should be passed in somewhere so that it can look elsewhere instead of body.