Description of bug or feature request

Given these two structures...

Structure 1: unhiding visually

<div style="display:none;"> <-- e.g., inside a disclosure widget -->
  <div aria-live="polite">chat message: hello</div>
</div>

Structure 2: unhiding from assistive technology

<div aria-hidden="true"> <-- e.g., behind a dialog -->
  <div aria-live="polite">chat message: hello</div>
</div>

...what should happen when I unhide the parent element, which is wrapped around an aria-live region?

Browsers are currently inconsistent. aria-live example with test results

I recommend we standardize on Firefox's current behavior in the above test results, because of these use cases:

aria-live inside of a disclosure widget. I have a visible chat log in a disclosure widget. I collapse the disclosure widget while I do other things in the page. When I expand the disclosure widget to continue chatting, I don't want the whole chat log to get announced again.
Dialog on the same page as aria-live. I have a visible chat log in the page. Elsewhere in the page, I trigger a modal dialog which hides most of the page content with aria-hidden, including the chat log. When I dismiss the dialog I don't want the whole chat log to get announced again.

Will this require a change to CORE-AAM?