refactor(core): Permit disabling autoDetect for zoneless fixture (ang…

…ular#55494)

The caveat here is that this needs to be done before creating the
fixture. There are some technical issues to overcome with disabling it
after it was already enabled, related to detaching from `ApplicationRef`
without other side effects.

PR Close angular#55494

packages/core/testing/src/component_fixture.ts

@@ -171,8 +171,12 @@ export abstract class ComponentFixture<T> {
  * `ApplicationRef.isStable`, and `autoDetectChanges` cannot be disabled.
  */
 export class ScheduledComponentFixture<T> extends ComponentFixture<T> {
+  private _autoDetect = inject(ComponentFixtureAutoDetect, {optional: true}) ?? true;
+
   initialize(): void {
-    this._appRef.attachView(this.componentRef.hostView);
+    if (this._autoDetect) {
+      this._appRef.attachView(this.componentRef.hostView);
+    }
   }
 
   override detectChanges(checkNoChanges: boolean = true): void {
@@ -189,7 +193,12 @@ export class ScheduledComponentFixture<T> extends ComponentFixture<T> {
 
   override autoDetectChanges(autoDetect?: boolean|undefined): void {
     if (!autoDetect) {
-      throw new Error('Cannot disable autoDetect when using the zoneless scheduler.');
+      throw new Error(
+          'Cannot disable autoDetect after it has been enabled when using the zoneless scheduler. ' +
+          'To disable autoDetect, add `{provide: ComponentFixtureAutoDetect, useValue: false}` to the TestBed providers.');
+    } else if (!this._autoDetect) {
+      this._autoDetect = autoDetect;
+      this._appRef.attachView(this.componentRef.hostView);
     }
     this.detectChanges();
   }