1. This requires exposing radiogroup's focusedItem property to C++.
Unfortunately, there's no existing equivalent in nsIDOMXULSelectControlItemElement.
radiogroup is the only element that needs this, so a new interface has been created for it.
2. Accessibility uses focusedItem instead of selectedItem when setting focus.
3. When an item is focused, accessibility needs to be notified.
This is done using a DOMMenuItemActive event.
Differential Revision: https://phabricator.services.mozilla.com/D15295
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extra : moz-landing-system : lando
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