fix(hydration): defer forwarded slot fallback hydration cleanup

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

@@ -9201,6 +9201,60 @@ describe('VDOM interop', () => {
     )
   })
 
+  test('hydrate forwarded slot fallback with nested component before parent close marker', async () => {
+    const data = ref('foo')
+    const { container } = await testWithVaporApp(
+      `<script setup>
+        const components = _components
+      </script>
+      <template>
+        <components.Child>
+          <template #foo><slot name="foo" /></template>
+        </components.Child>
+      </template>`,
+      {
+        Child: {
+          code: `<script setup>
+              const components = _components
+              const data = _data
+            </script>
+            <template>
+              <div>
+                <slot name="foo">
+                  <components.GrandChild
+                    v-if="data"
+                    :text="data"
+                  />
+                </slot>
+              </div>
+            </template>`,
+          vapor: true,
+        },
+        GrandChild: {
+          code: `<script setup>
+              defineProps(['text'])
+            </script>
+            <template>
+              <template v-if="text">
+                <span v-if="text">{{ text }}</span>
+              </template>
+            </template>`,
+          vapor: true,
+        },
+      },
+      data,
+    )
+
+    expect(`Hydration node mismatch`).not.toHaveBeenWarned()
+    expect(formatHtml(container.innerHTML)).toMatchInlineSnapshot(`
+      "<div>
+      <!--[-->
+      <!--[--><span>foo</span><!--if--><!--if--><!--if--><!--]-->
+      <!--slot--><!--]-->
+      </div>"
+    `)
+  })
+
   test('hydrate VDOM component returning Fragment', async () => {
     const data = ref('foo')
     const { container } = await testWithVaporApp(

packages/runtime-vapor/src/fragment.ts

@@ -499,8 +499,11 @@ export class DynamicFragment extends VaporFragment {
       let parentNode: Node | null
       let nextNode: Node | null
       if (forwardedSlot) {
-        parentNode = slotAnchor!.parentNode
-        nextNode = slotAnchor!.nextSibling
+        // Keep the forwarded slot close marker structural for parent cleanup,
+        // even though this fragment uses a runtime anchor after it.
+        const anchor = markHydrationAnchor(slotAnchor!)
+        parentNode = anchor.parentNode
+        nextNode = anchor.nextSibling
       } else {
         const node = findBlockNode(this.nodes)
         parentNode = node.parentNode
@@ -511,11 +514,13 @@ export class DynamicFragment extends VaporFragment {
       // Otherwise detached anchors could be observed too early by traversal
       // logic such as `findLastChild()`.
       queuePostFlushCb(() => {
+        const anchor =
+          nextNode && nextNode.parentNode === parentNode ? nextNode : null
         parentNode!.insertBefore(
           (this.anchor = markHydrationAnchor(
             __DEV__ ? createComment(this.anchorLabel!) : createTextNode(),
           )),
-          nextNode,
+          anchor,
         )
       })
     } finally {
@@ -566,6 +571,7 @@ export let currentEmptyFragment: DynamicFragment | null | undefined
 
 export class SlotFragment extends DynamicFragment {
   forwarded = false
+  deferredHydrationBoundary?: () => void
 
   constructor() {
     super(isHydrating || __DEV__ ? 'slot' : undefined, false, false)
@@ -579,6 +585,7 @@ export class SlotFragment extends DynamicFragment {
     let prevEndAnchor: Node | null = null
     let pushedEndAnchor = false
     let exitHydrationBoundary: (() => void) | undefined
+    let deferHydrationBoundary = false
     if (isHydrating) {
       locateHydrationNode()
       if (isComment(currentHydrationNode!, '[')) {
@@ -624,12 +631,24 @@ export class SlotFragment extends DynamicFragment {
       // once against the final block.
       if (isHydrating) {
         this.hydrate(render == null, true)
+        // Empty slots rendered while resolving an outer slot fallback can be
+        // filled by that fallback immediately after render() returns.
+        deferHydrationBoundary =
+          !!exitHydrationBoundary &&
+          currentEmptyFragment !== undefined &&
+          !isValidBlock(this.nodes)
       }
     } finally {
-      if (isHydrating && pushedEndAnchor) {
-        setCurrentSlotEndAnchor(prevEndAnchor)
+      if (isHydrating) {
+        if (pushedEndAnchor) {
+          setCurrentSlotEndAnchor(prevEndAnchor)
+        }
+        if (deferHydrationBoundary) {
+          this.deferredHydrationBoundary = exitHydrationBoundary
+        } else {
+          exitHydrationBoundary && exitHydrationBoundary()
+        }
       }
-      exitHydrationBoundary && exitHydrationBoundary()
     }
   }
 }
@@ -657,6 +676,15 @@ export function renderSlotFallback(
       frag.nodes[0] = [fallback() || []] as Block[]
     } else if (frag instanceof DynamicFragment) {
       frag.update(fallback)
+      if (isHydrating && frag instanceof SlotFragment) {
+        const deferredHydrationBoundary = frag.deferredHydrationBoundary
+        if (deferredHydrationBoundary) {
+          frag.deferredHydrationBoundary = undefined
+          // The fallback has now had a chance to hydrate the SSR nodes that
+          // originally belonged to the empty forwarded slot.
+          deferredHydrationBoundary()
+        }
+      }
     }
     return block
   }