fix(undo-redo): clearStack should also clear the previous item

libs/ngrx-toolkit/src/lib/with-undo-redo.spec.ts

@@ -210,5 +210,36 @@ describe('withUndoRedo', () => {
       expect(store.canUndo()).toBe(false);
       expect(store.canRedo()).toBe(false);
     })
+
+    it('cannot undo after clearing and setting a new value', fakeAsync(() => {
+      const Store = signalStore(
+        { providedIn: 'root' },
+        withState(testState),
+        withMethods((store) => ({
+          update: (value: string) => patchState(store, { test: value }),
+        })),
+        withUndoRedo({ keys: testKeys })
+      );
+
+      const store = TestBed.inject(Store);
+
+      store.update('Alan');
+      tick(1);
+
+      store.update('Gordon');
+      tick(1);
+
+      store.clearStack();
+      tick(1);
+
+      // After clearing the undo/redo stack, there is no previous item anymore.
+      // The following update becomes the first value.
+      // Since there is no other value before, it cannot be undone.
+      store.update('Hugh');
+      tick(1);
+
+      expect(store.canUndo()).toBe(false);
+      expect(store.canRedo()).toBe(false);
+    }));
   });
 });

libs/ngrx-toolkit/src/lib/with-undo-redo.ts

@@ -130,6 +130,7 @@ export function withUndoRedo<
       clearStack(): void {
         undoStack.splice(0);
         redoStack.splice(0);
+        previous = null;
         updateInternal();
       },
     })),