fix(focus): handle single radio button in FocusScope

packages/@react-aria/focus/src/FocusScope.tsx

@@ -15,6 +15,7 @@ import {
   getActiveElement,
   getEventTarget,
   getOwnerDocument,
+  getOwnerWindow,
   isAndroid,
   isChrome,
   isFocusable,
@@ -295,23 +296,37 @@ function shouldContainFocus(scopeRef: ScopeRef) {
   return true;
 }
 
-function isTabbableRadio(element: HTMLInputElement) {
-  if (element.checked) {
-    return true;
-  }
-  let radios: HTMLInputElement[] = [];
+function getRadiosInGroup(element: HTMLInputElement): HTMLInputElement[] {
   if (!element.form) {
-    radios = ([...getOwnerDocument(element).querySelectorAll(`input[type="radio"][name="${CSS.escape(element.name)}"]`)] as HTMLInputElement[]).filter(radio => !radio.form);
-  } else {
-    let radioList = element.form?.elements?.namedItem(element.name) as RadioNodeList;
-    radios = [...(radioList ?? [])] as HTMLInputElement[];
+    // Radio buttons outside a form - query the document
+    return Array.from(
+      getOwnerDocument(element).querySelectorAll<HTMLInputElement>(
+        `input[type="radio"][name="${CSS.escape(element.name)}"]`
+      )
+    ).filter(radio => !radio.form);
   }
-  if (!radios) {
-    return false;
+
+  // namedItem returns RadioNodeList (iterable) for 2+ elements, but a single Element for exactly 1.
+  // https://developer.mozilla.org/en-US/docs/Web/API/HTMLFormControlsCollection/namedItem
+  const radioList = element.form.elements.namedItem(element.name);
+  let ownerWindow = getOwnerWindow(element);
+  if (radioList instanceof ownerWindow.RadioNodeList) {
+    return Array.from(radioList).filter(
+      (el): el is HTMLInputElement => el instanceof ownerWindow.HTMLInputElement
+    );
   }
-  let anyChecked = radios.some(radio => radio.checked);
+  if (radioList instanceof ownerWindow.HTMLInputElement) {
+    return [radioList];
+  }
+  return [];
+}
 
-  return !anyChecked;
+function isTabbableRadio(element: HTMLInputElement): boolean {
+  if (element.checked) {
+    return true;
+  }
+  const radios = getRadiosInGroup(element);
+  return radios.length > 0 && !radios.some(radio => radio.checked);
 }
 
 function useFocusContainment(scopeRef: RefObject<Element[] | null>, contain?: boolean) {

packages/@react-aria/focus/test/FocusScope.test.js

@@ -1538,6 +1538,32 @@ describe('FocusScope', function () {
     expect(document.activeElement).toBe(getByTestId('button1'));
   });
 
+  it('handles forms with a single radio button without crashing', async function () {
+    // Regression test for https://github.com/adobe/react-spectrum/issues/9569
+    // form.elements.namedItem() returns Element (not RadioNodeList) for single elements
+    function Test() {
+      return (
+        <FocusScope contain>
+          <button data-testid="button1">First button</button>
+          <form>
+            <input type="radio" id="only" name="option" value="only" />
+            <label htmlFor="only">Only Option</label>
+          </form>
+          <button data-testid="button2">Second button</button>
+        </FocusScope>
+      );
+    }
+
+    let {getByTestId, getByRole} = render(<Test />);
+    let radio = getByRole('radio');
+    await user.tab();
+    expect(document.activeElement).toBe(getByTestId('button1'));
+    await user.tab();
+    expect(document.activeElement).toBe(radio);
+    await user.tab();
+    expect(document.activeElement).toBe(getByTestId('button2'));
+  });
+
   describe('nested focus scopes', function () {
     it('should make child FocusScopes the active scope regardless of DOM structure', function () {
       function ChildComponent(props) {