fix: post watcher not triggered (fix vuejs#12664)

Author: JuniorTour

.gitignore

@@ -12,6 +12,7 @@ packages/server-renderer/client-plugin.js
 packages/template-compiler/build.js
 packages/template-compiler/browser.js
 .vscode
+.idea
 dist
 temp
 types/v3-generated.d.ts

src/core/observer/scheduler.ts

@@ -156,7 +156,7 @@ function callActivatedHooks(queue) {
  */
 export function queueWatcher(watcher: Watcher) {
   const id = watcher.id
-  if (has[id] != null) {
+  if (id !== Infinity && has[id] != null) {
     return
   }
 

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

@@ -672,6 +672,51 @@ describe('api: watch', () => {
     await nextTick()
     expect(dom!.tagName).toBe('P')
   })
+  
+  // #12664
+  it('flush: post watchers from 2 components should both fire after template refs updated', async () => {
+    let appPostWatcherTrigger = false
+    let childComponentPostWatcherTrigger = false
+
+    const Child = {
+      setup() {
+        const el = ref();
+        watch(
+          el,
+          () => {
+            childComponentPostWatcherTrigger = true
+          },
+          {flush: 'post'}
+        )
+        return { el };
+      },
+      template: `<div><span ref="el">hello child</span></div>`
+    }
+    const App = {
+      components: { Child },
+      setup() {
+        const el = ref();
+        watch(
+          el,
+          () => {
+            appPostWatcherTrigger = true
+          },
+          {flush: 'post'}
+        )
+        return { el };
+      },
+      template: `<div><Child /><span ref="el">hello app1</span></div>`
+    }
+
+    const container = document.createElement('div')
+    const root = document.createElement('div')
+    container.appendChild(root)
+    new Vue(App).$mount(root)
+
+    await nextTick()
+    expect(appPostWatcherTrigger).toBe(true)
+    expect(childComponentPostWatcherTrigger).toBe(true)
+  })
 
   it('deep', async () => {
     const state = reactive({