test: apiWatch #12967

package.json

@@ -45,6 +45,7 @@
     "build:types": "rimraf temp && tsc --declaration --emitDeclarationOnly --outDir temp && api-extractor run && api-extractor run -c packages/compiler-sfc/api-extractor.json",
     "test": "npm run ts-check && npm run test:types && npm run test:unit && npm run test:e2e && npm run test:ssr && npm run test:sfc",
     "test:unit": "vitest run test/unit",
+    "test:watch": "vitest run test/unit/features/v3/apiWatch.spec.ts",
     "test:ssr": "npm run build:ssr && vitest run server-renderer",
     "test:sfc": "vitest run compiler-sfc",
     "test:e2e": "npm run build -- full-prod,server-renderer-basic && vitest run test/e2e",

test/unit/features/v3/apiWatch.spec.ts

@@ -1200,4 +1200,15 @@ describe('api: watch', () => {
     expect(parentSpy).toHaveBeenCalledTimes(1)
     expect(childSpy).toHaveBeenCalledTimes(1)
   })
+
+  // #12967
+  test('trigger when adding new property with Vue.set', async () => {
+    const spy = vi.fn()
+    const r = reactive({ exist: 5 })
+    watch(r, spy, { deep: true })
+    set(r, 'add', 1)
+
+    await nextTick()
+    expect(spy).toHaveBeenCalledTimes(1)
+  })
 })