From 898964c3688e3593d9aabbb09b4849e58a208ce8 Mon Sep 17 00:00:00 2001
From: Alex Snezhko <alexsnezhko89@gmail.com>
Date: Mon, 6 Oct 2025 21:23:07 -0700
Subject: [PATCH] fix(runtime-vapor): fallthrough attrs with comments in
 template root

---
 .../__tests__/componentAttrs.spec.ts          | 125 ++++++++++++++++++
 packages/runtime-vapor/src/component.ts       |  30 ++++-
 2 files changed, 150 insertions(+), 5 deletions(-)

diff --git a/packages/runtime-vapor/__tests__/componentAttrs.spec.ts b/packages/runtime-vapor/__tests__/componentAttrs.spec.ts
index 1f43ebba8c0..d513a0bb88c 100644
--- a/packages/runtime-vapor/__tests__/componentAttrs.spec.ts
+++ b/packages/runtime-vapor/__tests__/componentAttrs.spec.ts
@@ -57,6 +57,131 @@ describe('attribute fallthrough', () => {
     expect(host.innerHTML).toBe('<div id="b">2</div>')
   })
 
+  it('should allow attrs to fallthrough on component with comment at root', async () => {
+    const t0 = template('<!--comment-->')
+    const t1 = template('<div>')
+    const { component: Child } = define({
+      props: ['foo'],
+      setup(props: any) {
+        const n0 = t0()
+        const n1 = t1()
+        renderEffect(() => setElementText(n1, props.foo))
+        return [n0, n1]
+      },
+    })
+
+    const foo = ref(1)
+    const id = ref('a')
+    const { host } = define({
+      setup() {
+        return createComponent(
+          Child,
+          {
+            foo: () => foo.value,
+            id: () => id.value,
+          },
+          null,
+          true,
+        )
+      },
+    }).render()
+    expect(host.innerHTML).toBe('<!--comment--><div id="a">1</div>')
+
+    foo.value++
+    await nextTick()
+    expect(host.innerHTML).toBe('<!--comment--><div id="a">2</div>')
+
+    id.value = 'b'
+    await nextTick()
+    expect(host.innerHTML).toBe('<!--comment--><div id="b">2</div>')
+  })
+
+  it('should allow attrs to fallthrough on component with single-element array root', async () => {
+    const t0 = template('<div>')
+    const { component: Child } = define({
+      props: ['foo'],
+      setup(props: any) {
+        const n0 = t0()
+        renderEffect(() => setElementText(n0, props.foo))
+        return [n0]
+      },
+    })
+
+    const foo = ref(1)
+    const id = ref('a')
+    const { host } = define({
+      setup() {
+        return createComponent(
+          Child,
+          {
+            foo: () => foo.value,
+            id: () => id.value,
+          },
+          null,
+          true,
+        )
+      },
+    }).render()
+    expect(host.innerHTML).toBe('<div id="a">1</div>')
+
+    foo.value++
+    await nextTick()
+    expect(host.innerHTML).toBe('<div id="a">2</div>')
+
+    id.value = 'b'
+    await nextTick()
+    expect(host.innerHTML).toBe('<div id="b">2</div>')
+  })
+
+  it('should not allow attrs to fallthrough on component with multiple roots', async () => {
+    const t0 = template('<span>')
+    const t1 = template('<div>')
+    const { component: Child } = define({
+      props: ['foo'],
+      setup(props: any) {
+        const n0 = t0()
+        const n1 = t1()
+        renderEffect(() => setElementText(n1, props.foo))
+        return [n0, n1]
+      },
+    })
+
+    const foo = ref(1)
+    const id = ref('a')
+    const { host } = define({
+      setup() {
+        return createComponent(
+          Child,
+          {
+            foo: () => foo.value,
+            id: () => id.value,
+          },
+          null,
+          true,
+        )
+      },
+    }).render()
+    expect(host.innerHTML).toBe('<span></span><div>1</div>')
+  })
+
+  it('should not allow attrs to fallthrough on component with single comment root', async () => {
+    const t0 = template('<!--comment-->')
+    const { component: Child } = define({
+      setup() {
+        const n0 = t0()
+        return [n0]
+      },
+    })
+
+    const id = ref('a')
+    const { host } = define({
+      setup() {
+        return createComponent(Child, { id: () => id.value }, null, true)
+      },
+    }).render()
+    expect(host.innerHTML).toBe('<!--comment-->')
+  })
+
   it('should not fallthrough if explicitly pass inheritAttrs: false', async () => {
     const t0 = template('<div>', true)
     const { component: Child } = define({
diff --git a/packages/runtime-vapor/src/component.ts b/packages/runtime-vapor/src/component.ts
index 08fd881e959..7067c8f4e6a 100644
--- a/packages/runtime-vapor/src/component.ts
+++ b/packages/runtime-vapor/src/component.ts
@@ -34,7 +34,13 @@ import {
   setActiveSub,
   unref,
 } from '@vue/reactivity'
-import { EMPTY_OBJ, invokeArrayFns, isFunction, isString } from '@vue/shared'
+import {
+  EMPTY_OBJ,
+  invokeArrayFns,
+  isArray,
+  isFunction,
+  isString,
+} from '@vue/shared'
 import {
   type DynamicPropsSource,
   type RawProps,
@@ -255,7 +261,7 @@ export function createComponent(
     component.inheritAttrs !== false &&
     Object.keys(instance.attrs).length
   ) {
-    const el = getRootElement(instance)
+    const el = getRootElement(instance.block)
     if (el) {
       renderEffect(() => {
         isApplyingFallthroughProps = true
@@ -579,9 +585,7 @@ export function getExposed(
   }
 }
 
-function getRootElement({
-  block,
-}: VaporComponentInstance): Element | undefined {
+function getRootElement(block: Block): Element | undefined {
   if (block instanceof Element) {
     return block
   }
@@ -592,4 +596,20 @@ function getRootElement({
       return nodes
     }
   }
+
+  if (isArray(block)) {
+    let singleRoot: Element | undefined
+    for (const b of block) {
+      if (b instanceof Comment) {
+        continue
+      }
+      const thisRoot = getRootElement(b)
+      // only return root if there is exactly one eligible root in the array
+      if (!thisRoot || singleRoot) {
+        return
+      }
+      singleRoot = thisRoot
+    }
+    return singleRoot
+  }
 }