refactor(multiple): stabilize focus state when active item is deleted (#33199)

- Calls setDefaultState instead of dropping focus when active item is removed.
- Clears stale reference to prevent memory leaks.
- Adds focus stability unit tests for listbox and tree.

(cherry picked from commit cd62949)

Author: tjshiu

src/aria/listbox/listbox.spec.ts

@@ -822,6 +822,23 @@ describe('Listbox', () => {
       expect(listboxInstance.value()).toEqual([]);
     });
   });
+
+  describe('item mutations and focus stability', () => {
+    it('should recover focus by shifting to the default state if the active option is removed', async () => {
+      setupListbox({focusMode: 'activedescendant'});
+      click(2);
+      expect(listboxElement.getAttribute('aria-activedescendant')).toBe(optionElements[2].id);
+
+      const testComponent = fixture.componentInstance as ListboxExample;
+      const updatedOptions = testComponent.options().filter(o => o.value !== 2);
+      testComponent.options.set(updatedOptions);
+      fixture.detectChanges();
+      await waitForMicrotasks();
+      defineTestVariables(fixture);
+
+      expect(listboxElement.getAttribute('aria-activedescendant')).toBe(optionElements[0].id);
+    });
+  });
 });
 
 interface TestOption {

src/aria/listbox/listbox.ts

@@ -182,8 +182,9 @@ export class Listbox<V> implements OnDestroy {
         const items = inputs.items();
         const activeItem = untracked(() => inputs.activeItem());
 
-        if (!items.some(i => i === activeItem) && activeItem) {
+        if (activeItem && !items.some(i => i === activeItem)) {
           this._pattern.listBehavior.unfocus();
+          this._pattern.setDefaultState();
         }
       },
     });

src/aria/tree/tree.spec.ts

@@ -1556,6 +1556,25 @@ describe('Tree', () => {
       });
     }
   });
+
+  describe('item mutations and focus stability', () => {
+    it('should recover focus by shifting to the default state if the active item is removed', async () => {
+      setupTestTree();
+      updateTree({focusMode: 'activedescendant'});
+
+      const vegetablesEl = getTreeItemElementByValue('vegetables')!;
+      click(vegetablesEl);
+      expect(getFocusedTreeItemValue()).toBe('vegetables');
+
+      const updatedNodes = testComponent.nodes().filter(n => n.value !== 'vegetables');
+      testComponent.nodes.set(updatedNodes);
+      fixture.detectChanges();
+      await waitForMicrotasks();
+      defineTestVariables();
+
+      expect(getFocusedTreeItemValue()).toBe('fruits');
+    });
+  });
 });
 
 interface TestTreeNode<V = string> {

src/aria/tree/tree.ts

@@ -192,8 +192,9 @@ export class Tree<V> implements OnDestroy {
         const items = inputs.items();
         const activeItem = untracked(() => inputs.activeItem());
 
-        if (!items.some(i => i === activeItem) && activeItem) {
+        if (activeItem && !items.some(i => i === activeItem)) {
           this._pattern.treeBehavior.unfocus();
+          this._pattern.setDefaultState();
         }
       },
     });