Implement focus fixup rule

• Component: DOM
• Status: RESOLVED FIXED

[Tim Nguyen (:ntim)]

https://html.spec.whatwg.org/multipage/interaction.html#focus-fixup-rule

Focus fixup rule: When the designated focused area of the document is removed from that Document in some way (e.g. it stops being a focusable area, it is removed from the DOM, it becomes inert, etc.), designate the Document's viewport to be the new focused area of the document.

[Tim Nguyen (:ntim)]

Adding this seems to work:

```
        if (auto* activeElement = document.activeElement()) {
            if (!activeElement->isFocusable())
                document.setFocusedElement(nullptr);
        }
```

I put it in the RenderingUpdateStep::FlushAutofocusCandidates block, but that's probably not the right place (not sure which one is?).

One thing that's broken is interaction with shadow DOM: shadow-dom/focus/focus-pseudo-on-shadow-host-1.html fails. This is precisely because activeElement->isFocusable() doesn't take in account contents of the shadow DOM, and wrongly clears focus in this case. Though I expect that's not too hard to fix.

[Tim Nguyen (:ntim)]

imported/w3c/web-platform-tests/inert/dynamic-inert-on-focused-element.tentative.html tests this functionality.

I'm actually a bit surprised there aren't more tests (e.g. moving out of a focusable area, disabled attribute, etc.)

[Radar WebKit Bug Importer]

<rdar://problem/89902824>

[Ahmad Saleem]

Safari TP 151 is still failing these tests:

https://wpt.fyi/results/inert/dynamic-inert-on-focused-element.html?label=master&label=experimental&aligned&view=subtest&q=dynamic-inert-on-focused-element

[Tim Nguyen (:ntim)]

Pull request: https://github.com/WebKit/WebKit/pull/6069

[Ryosuke Niwa]

Pull request: https://github.com/WebKit/WebKit/pull/9835

[EWS]

Committed 260067@main (71bc5343fd7b): <https://commits.webkit.org/260067@main>

Reviewed commits have been landed. Closing PR #9835 and removing active labels.