fix(watch): should work normal w/ sync flush in ssr

packages/runtime-core/src/apiWatch.ts

@@ -280,7 +280,7 @@ function doWatch(
   }
 
   // in SSR there is no need to setup an actual effect, and it should be noop
-  // unless it's eager
+  // unless it's eager or sync flush
   if (__SSR__ && isInSSRComponentSetup) {
     // we will also not call the invalidate callback (+ runner is not set up)
     onCleanup = NOOP
@@ -293,7 +293,9 @@ function doWatch(
         onCleanup
       ])
     }
-    return NOOP
+    if (flush !== 'sync') {
+      return NOOP
+    }
   }
 
   let oldValue = isMultiSource ? [] : INITIAL_WATCHER_VALUE

packages/server-renderer/__tests__/ssrWatch.spec.ts

@@ -0,0 +1,43 @@
+import { createSSRApp, defineComponent, h, watch, ref } from 'vue'
+import { renderToString } from '../src/renderToString'
+
+describe('ssr: watch', () => {
+  // #6013
+  test('should work w/ flush:sync', async () => {
+    const App = defineComponent(() => {
+      const count = ref(0)
+      let msg = ''
+      watch(
+        count,
+        () => {
+          msg = 'hello world'
+        },
+        { flush: 'sync' }
+      )
+      count.value = 1
+      expect(msg).toBe('hello world')
+      return () => h('div', null, msg)
+    })
+    const app = createSSRApp(App)
+    const html = await renderToString(app)
+    expect(html).toMatch('hello world')
+  })
+  test('should work w/ flush:sync', async () => {
+    const App = defineComponent(() => {
+      const count = ref(0)
+      let msg = 'abc'
+      watch(
+        count,
+        () => {
+          msg = 'hello world'
+        },
+        { flush: 'sync' }
+      )
+      expect(msg).toBe('abc')
+      return () => h('div', null, msg)
+    })
+    const app = createSSRApp(App)
+    const html = await renderToString(app)
+    expect(html).toMatch('abc')
+  })
+})