fix(focus): handle single radio button in FocusScope

[jtstothard]

Summary

Fixes #9569

FocusScope crashes with TypeError: (intermediate value) is not iterable when containing a form with exactly one unchecked radio button.

Root Cause

Per the DOM spec, form.elements.namedItem() returns:

• RadioNodeList (iterable) for 2+ elements with the same name
• Element (NOT iterable) for exactly 1 element
• null for no elements

The isTabbableRadio function assumed it always returns RadioNodeList and tried to spread it, causing the crash.

Solution

Refactored into two pure functions with proper type narrowing:

function getRadiosInGroup(element: HTMLInputElement): HTMLInputElement[] {
  // ...
  const radioList = element.form.elements.namedItem(element.name);
  if (radioList instanceof RadioNodeList) {
    return Array.from(radioList).filter(
      (el): el is HTMLInputElement => el instanceof HTMLInputElement
    );
  }
  if (radioList instanceof HTMLInputElement) {
    return [radioList];
  }
  return [];
}

function isTabbableRadio(element: HTMLInputElement): boolean {
  if (element.checked) {
    return true;
  }
  const radios = getRadiosInGroup(element);
}

Key improvements:

• Extracted getRadiosInGroup as a pure function with early returns
• No let declarations - only const
• No type casting - uses instanceof checks and type predicates
• Explicit return type annotations
• isTabbableRadio is now a simple composition

Testing

• Added regression test for single radio button scenario
• All 58 FocusScope tests pass

Checklist

• Tests added
• Lint passes
• References issue FocusScope crashes with single radio button in form #9569

[snowystinger]

Thanks, one change I'm making so that we don't regress on our IFrame support

[snowystinger]

apologies, it didn't like that i spanned a non-touched line for some reason

[jtstothard]

@snowystinger thanks for the review, is there any way to add the ready to review tag so others review too?

[snowystinger]

thanks for the reminder, it's actually just a tag for myself, has no bearing on when others will review it