fix(material/menu): lazy content not detached after animation (#30301)

Fixes a regression after #30148 where we no longer detached the lazy content panel, causing its items to be retained until the next open when they get destroyed and re-created.

(cherry picked from commit 86a96c9)

src/material/menu/menu-trigger.ts

@@ -368,10 +368,14 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
     // Note that we don't wait for the animation to finish if another trigger took
     // over the menu, because the panel will end up empty which looks glitchy.
     if (menu instanceof MatMenu && this._ownsMenu(menu)) {
-      this._pendingRemoval = menu._animationDone.pipe(take(1)).subscribe(() => overlayRef.detach());
+      this._pendingRemoval = menu._animationDone.pipe(take(1)).subscribe(() => {
+        overlayRef.detach();
+        menu.lazyContent?.detach();
+      });
       menu._setIsOpen(false);
     } else {
       overlayRef.detach();
+      menu?.lazyContent?.detach();
     }
 
     if (menu && this._ownsMenu(menu)) {

src/material/menu/menu.spec.ts

@@ -15,9 +15,11 @@ import {ScrollDispatcher, ViewportRuler} from '@angular/cdk/scrolling';
 import {
   ChangeDetectionStrategy,
   Component,
+  Directive,
   ElementRef,
   EventEmitter,
   Input,
+  OnDestroy,
   Output,
   Provider,
   QueryList,
@@ -1219,6 +1221,40 @@ describe('MatMenu', () => {
         .toBe(true);
     }));
 
+    it('should detach the lazy content when the menu is closed', fakeAsync(() => {
+      let destroyCount = 0;
+
+      // Note: for some reason doing `spyOn(item, 'ngOnDestroy')` doesn't work, even though a
+      // `console.log` shows that the `ngOnDestroy` gets called. We work around it with a custom
+      // directive that increments a counter.
+      @Directive({selector: '[mat-menu-item]', standalone: false})
+      class DestroyChecker implements OnDestroy {
+        ngOnDestroy(): void {
+          destroyCount++;
+        }
+      }
+
+      const fixture = createComponent(SimpleLazyMenu, undefined, [DestroyChecker]);
+      fixture.detectChanges();
+      fixture.componentInstance.trigger.openMenu();
+      fixture.detectChanges();
+      tick(500);
+      fixture.detectChanges();
+
+      expect(fixture.componentInstance.items.length).toBe(2);
+      expect(destroyCount).toBe(0);
+
+      fixture.componentInstance.trigger.closeMenu();
+      fixture.detectChanges();
+      tick(500);
+      fixture.detectChanges();
+
+      expect(fixture.componentInstance.items.length)
+        .withContext('Expected items to be removed from query list')
+        .toBe(0);
+      expect(destroyCount).withContext('Expected ngOnDestroy to have been called').toBe(2);
+    }));
+
     it('should focus the first menu item when opening a lazy menu via keyboard', async () => {
       const fixture = createComponent(SimpleLazyMenu);
       fixture.autoDetectChanges();