fix(runtime-vapor): render slot fallback if slot content is not a valid block

close #13668

packages/runtime-vapor/__tests__/componentSlots.spec.ts

@@ -502,5 +502,64 @@ describe('component: slots', () => {
       await nextTick()
       expect(host.innerHTML).toBe('<div><h1></h1><!--slot--></div>')
     })
+
+    test('render fallback when slot content is not valid', async () => {
+      const Child = {
+        setup() {
+          return createSlot('default', null, () =>
+            document.createTextNode('fallback'),
+          )
+        },
+      }
+
+      const { html } = define({
+        setup() {
+          return createComponent(Child, null, {
+            default: () => {
+              return template('<!--comment-->')()
+            },
+          })
+        },
+      }).render()
+
+      expect(html()).toBe('fallback<!--slot-->')
+    })
+
+    test('render fallback when v-if condition is false', async () => {
+      const Child = {
+        setup() {
+          return createSlot('default', null, () =>
+            document.createTextNode('fallback'),
+          )
+        },
+      }
+
+      const toggle = ref(false)
+
+      const { html } = define({
+        setup() {
+          return createComponent(Child, null, {
+            default: () => {
+              return createIf(
+                () => toggle.value,
+                () => {
+                  return document.createTextNode('content')
+                },
+              )
+            },
+          })
+        },
+      }).render()
+
+      expect(html()).toBe('fallback<!--if--><!--slot-->')
+
+      toggle.value = true
+      await nextTick()
+      expect(html()).toBe('content<!--if--><!--slot-->')
+
+      toggle.value = false
+      await nextTick()
+      expect(html()).toBe('fallback<!--if--><!--slot-->')
+    })
   })
 })

packages/runtime-vapor/src/componentSlots.ts

@@ -1,5 +1,11 @@
 import { EMPTY_OBJ, NO, hasOwn, isArray, isFunction } from '@vue/shared'
-import { type Block, type BlockFn, DynamicFragment, insert } from './block'
+import {
+  type Block,
+  type BlockFn,
+  DynamicFragment,
+  insert,
+  isValidBlock,
+} from './block'
 import { rawPropsProxyHandlers } from './componentProps'
 import { currentInstance, isRef } from '@vue/runtime-dom'
 import type { LooseRawProps, VaporComponentInstance } from './component'
@@ -126,6 +132,7 @@ export function createSlot(
     const renderSlot = () => {
       const slot = getSlot(rawSlots, isFunction(name) ? name() : name)
       if (slot) {
+        fragment.fallback = fallback
         // create and cache bound version of the slot to make it stable
         // so that we avoid unnecessary updates if it resolves to the same slot
         fragment.update(
@@ -133,7 +140,13 @@ export function createSlot(
             (slot._bound = () => {
               const slotContent = slot(slotProps)
               if (slotContent instanceof DynamicFragment) {
-                slotContent.fallback = fallback
+                // render fallback if slot content is not a valid block
+                if (
+                  (slotContent.fallback = fallback) &&
+                  !isValidBlock(slotContent.nodes)
+                ) {
+                  slotContent.update(fallback)
+                }
               }
               return slotContent
             }),