[Modal] Never hide an aria-hidden ancestor of the modal

ariaHiddenSiblings skipped hiding a container child if it contained the
modal, so a non-portaled modal (disablePortal) whose DOM node lives
inside one of the container's children is no longer rendered inside an
aria-hidden ancestor. Previously the container's children were hidden
unconditionally, making such modals inaccessible to assistive tech.

Author: atharv-sys32

packages/mui-material/src/Modal/ModalManager.test.ts

@@ -350,6 +350,20 @@ describe('ModalManager', () => {
       expect(modal2).not.toBeInaccessible();
     });
 
+    it('should not add aria-hidden to container siblings that contain the modal', () => {
+      // Simulates a non-portaled modal (disablePortal) whose DOM node is nested
+      // inside one of the container's children. Hiding that child would make the
+      // modal itself inaccessible.
+      const nestedParent = document.createElement('div');
+      const nestedModalRef = document.createElement('div');
+      nestedParent.appendChild(nestedModalRef);
+      container2.appendChild(nestedParent);
+
+      modalManager.add({ ...getDummyModal(), modalRef: nestedModalRef }, container2);
+
+      expect(nestedParent).not.toBeInaccessible();
+    });
+
     it('should add aria-hidden to container siblings', () => {
       const secondSibling = document.createElement('input');
       container2.appendChild(secondSibling);

packages/mui-material/src/Modal/ModalManager.ts

@@ -66,7 +66,11 @@ function ariaHiddenSiblings(
   [].forEach.call(container.children, (element: Element) => {
     const isNotExcludedElement = !blacklist.includes(element);
     const isNotForbiddenElement = !isAriaHiddenForbiddenOnElement(element);
-    if (isNotExcludedElement && isNotForbiddenElement) {
+    // Never hide an element that contains the modal, otherwise the modal itself
+    // becomes inaccessible. This happens when the modal is not portaled into the
+    // container, for example when `disablePortal` is used.
+    const isNotModalAncestor = currentElement ? !element.contains(currentElement) : true;
+    if (isNotExcludedElement && isNotForbiddenElement && isNotModalAncestor) {
       ariaHidden(element, hide);
     }
   });