AX: aria-activedescendant does not work in an iframe

• Component: Accessibility
• Status: NEW ---

See example: https://codepen.io/achint-singh/pen/rNKxVed

In codepen, the example is placed within an iframe. When placing this example outside of an iframe, it does work, such as using debug mode.

Steps to reproduce:
1. Turn on VO in the example.
2. Click the button using enter key, and navigate through the list using the down arrow key.

Notice that the VO focus ring is not moving alongside the dropdown selector, and is not reading out the options. 
The expected behavior is that using the down arrow key should simultaneously move the VO focus ring as well as the dropdown selector and read the options.

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This is also broken in the Safari Technology Preview.
In Safari 15.5, this code works as expected (in and out of the iframe). It also works with Chrome+VO, and Duck Duck Go+VO.

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This example also demonstrates an aria-selected issue, but that is part of a different bug.

[Radar WebKit Bug Importer]

<rdar://problem/101769043>

[Tyler Wilcock]

This should be fixed by:

https://bugs.webkit.org/show_bug.cgi?id=224582 (AX: `aria-activedescendant` doesn't work with a standard "listbox" pattern)

That change is in the latest macOS 13.3 beta (https://developer.apple.com/news/releases/?id=02162023c).