fix(core): clean up dehydrated views during HMR component replacement

During HMR, `recreateLView()` destroys the old LView and removes its
DOM nodes, but never cleans up dehydrated view DOM nodes stored in
`LContainer[DEHYDRATED_VIEWS]`. These are SSR-rendered DOM nodes
preserved by Angular's hydration system. When the new view renders,
both the old dehydrated DOM and the new DOM coexist, causing visible
duplication (e.g. `<app-shell>` header/footer appearing twice).

Call `cleanupLView` from the hydration cleanup module after
`destroyLView` and before `removeViewFromDOM` to remove any remaining
dehydrated DOM nodes before the replacement view is rendered.

Fixes #66503

packages/core/src/hydration/cleanup.ts

@@ -107,7 +107,7 @@ export function cleanupLContainer(lContainer: LContainer) {
  * Walks over `LContainer`s and components registered within
  * this LView and invokes dehydrated views cleanup function for each one.
  */
-function cleanupLView(lView: LView) {
+export function cleanupLView(lView: LView) {
   cleanupI18nHydrationData(lView);
 
   const tView = lView[TVIEW];

packages/core/src/render3/hmr.ts

@@ -34,6 +34,7 @@ import {
   TVIEW,
 } from './interfaces/view';
 import {assertTNodeType} from './node_assert';
+import {cleanupLView as cleanupDehydratedLView} from '../hydration/cleanup';
 import {destroyLView, removeViewFromDOM} from './node_manipulation';
 import {RendererFactory} from './interfaces/renderer';
 import {NgZone} from '../zone';
@@ -279,6 +280,12 @@ function recreateLView(
     // Destroy the detached LView.
     destroyLView(lView[TVIEW], lView);
 
+    // Clean up any dehydrated views left over from SSR hydration.
+    // Neither destroyLView nor removeViewFromDOM handle DOM nodes
+    // stored in LContainer[DEHYDRATED_VIEWS], which causes duplicated
+    // content when the view is re-rendered during HMR.
+    cleanupDehydratedLView(lView);
+
     // Always force the creation of a new renderer to ensure state captured during construction
     // stays consistent with the new component definition by clearing any old ached factories.
     const rendererFactory = lView[ENVIRONMENT].rendererFactory;

packages/core/test/acceptance/hmr_spec.ts

@@ -39,6 +39,11 @@ import {clearTranslations, loadTranslations} from '@angular/localize';
 import {computeMsgId} from '@angular/compiler';
 import {EVENT_MANAGER_PLUGINS} from '@angular/platform-browser';
 import {ComponentType} from '../../src/render3';
+import {getComponentLView} from '../../src/render3/util/discovery_utils';
+import {DEHYDRATED_VIEWS} from '../../src/render3/interfaces/container';
+import {HEADER_OFFSET, TVIEW} from '../../src/render3/interfaces/view';
+import {isLContainer} from '../../src/render3/interfaces/type_checks';
+import {NUM_ROOT_NODES} from '../../src/hydration/interfaces';
 import {isNode} from '@angular/private/testing';
 
 describe('hot module replacement', () => {
@@ -2075,6 +2080,65 @@ describe('hot module replacement', () => {
     });
   });
 
+  it('should clean up dehydrated views from LContainers during HMR', () => {
+    const initialMetadata: Component = {
+      selector: 'child-cmp',
+      template: '@if (true) { <div>Initial</div> }',
+    };
+
+    @Component(initialMetadata)
+    class ChildCmp {}
+
+    @Component({
+      imports: [ChildCmp],
+      template: '<child-cmp/>',
+    })
+    class RootCmp {}
+
+    const fixture = TestBed.createComponent(RootCmp);
+    fixture.detectChanges();
+
+    const childEl = fixture.nativeElement.querySelector('child-cmp')!;
+    expectHTML(fixture.nativeElement, '<child-cmp><div>Initial</div></child-cmp>');
+
+    // Simulate SSR dehydrated views by injecting fake dehydrated DOM nodes
+    // into the LContainer's DEHYDRATED_VIEWS slot. During SSR hydration,
+    // Angular stores references to server-rendered DOM in this slot.
+    const childLView = getComponentLView(childEl);
+    const tView = childLView[TVIEW];
+
+    // Create fake dehydrated DOM content that simulates SSR remnants.
+    // Insert before existing content so the node has a nextSibling,
+    // which removeDehydratedView validates in dev mode.
+    const dehydratedNode = document.createElement('div');
+    dehydratedNode.textContent = 'SSR ghost';
+    childEl.insertBefore(dehydratedNode, childEl.firstChild);
+
+    // Find the LContainer created by the @if and inject dehydrated views.
+    for (let i = HEADER_OFFSET; i < tView.bindingStartIndex; i++) {
+      if (isLContainer(childLView[i])) {
+        childLView[i][DEHYDRATED_VIEWS] = [
+          {firstChild: dehydratedNode, data: {[NUM_ROOT_NODES]: 1}},
+        ];
+        break;
+      }
+    }
+
+    // Verify the dehydrated node is present in the DOM.
+    expect(childEl.innerHTML).toContain('SSR ghost');
+
+    // Trigger HMR replacement.
+    replaceMetadata(ChildCmp, {
+      ...initialMetadata,
+      template: '@if (true) { <div>Replaced</div> }',
+    });
+    fixture.detectChanges();
+
+    // After HMR, dehydrated DOM nodes should have been cleaned up — no duplication.
+    expect(childEl.innerHTML).not.toContain('SSR ghost');
+    expectHTML(fixture.nativeElement, '<child-cmp><div>Replaced</div></child-cmp>');
+  });
+
   // Testing utilities
 
   // Field that we'll monkey-patch onto DOM elements that were created