Bug 255507

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.