fix: trigger when adding new property with Vue.seta and piWatch test

Author: xiaowei-one

src/v3/apiWatch.ts

@@ -218,8 +218,17 @@ function doWatch(
       })
   } else if (isFunction(source)) {
     if (cb) {
-      // getter with cb
-      getter = () => call(source, WATCHER_GETTER)
+      const ob = call(source, WATCHER_GETTER)
+      if (isReactive(ob)){
+        getter = () => {
+          ;(ob as any).__ob__.dep.depend()
+          return ob
+        }
+        deep = true
+      } else {
+        // getter with cb
+        getter = () => ob
+      }
     } else {
       // no cb -> simple effect
       getter = () => {

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)
+  })
 })