AX: showModal() does not include slotted content as "focusable"

• Component: Accessibility
• Status: NEW ---

[Westbrook]

When open a "modal" `<dialog>` via the `.showModal()` method, the flat tree is not used to calculate the "first focusable" element and rather than passing focus into content that should otherwise receive it, the focus is passed the `<dialog>` element.

https://codepen.io/Westbrook/pen/VwEjLEo

In the above demo, see the `<input type="text" />` that is slotted into the `<dialog>` element present in the shadow root created on the `<section>` element. When clicking the "Open slotted dialog" button, a visitor would expect to see focus passed into the `<input>` element directly due to the use of `.showModal()`. This is based on the way that clicking "Open flattened dialog" focus is accurately passed in the `<input type="text" />` within the `<dialog>` sharing the same DOM tree.

[Radar WebKit Bug Importer]

<rdar://problem/108127467>

[Westbrook]

This same issue exists in Firefox 111 (issue tracked via https://bugzilla.mozilla.org/show_bug.cgi?id=1828398), but behaves as expected in Chrome 111.