From f59914d1503645a9bb19663c94f5ed9722dba273 Mon Sep 17 00:00:00 2001
From: Valentin Yushkevich <valiantsinyushkevich@gmail.com>
Date: Thu, 1 Oct 2026 12:06:27 +0300
Subject: [PATCH] fix(runtime-vapor): only merge fallthrough attrs into a root
 component's props when there are some

---
 .../__tests__/componentProps.spec.ts          | 36 +++++++++++++++++++
 packages/runtime-vapor/src/component.ts       | 13 ++++---
 packages/runtime-vapor/src/componentProps.ts  | 19 ++++++++--
 3 files changed, 61 insertions(+), 7 deletions(-)

diff --git a/packages/runtime-vapor/__tests__/componentProps.spec.ts b/packages/runtime-vapor/__tests__/componentProps.spec.ts
index 9a0383fa21f..d4ce8fc47eb 100644
--- a/packages/runtime-vapor/__tests__/componentProps.spec.ts
+++ b/packages/runtime-vapor/__tests__/componentProps.spec.ts
@@ -3,6 +3,8 @@
 import {
   createApp,
   currentInstance,
+  defineComponent,
+  h,
   inject,
   isShallow,
   markRaw,
@@ -2606,4 +2608,38 @@ describe('component: props', () => {
       app.unmount()
     })
   })
+
+  test('empty fallthrough attrs are not merged into the root props', async () => {
+    const child = `<script setup>
+      defineProps(['style'])
+    </script><template><i>{{ typeof $props.style }}</i></template>`
+    const VdomChild = defineComponent({
+      props: ['style'],
+      setup: props => () => h('i', typeof props.style),
+    })
+    const wrapper = (child: string, bind = '') => `<script setup>
+      const data = _data
+      const components = _components
+      defineEmits(['ready'])
+    </script><template><components.${child} :style="data.style" ${bind} /></template>`
+    const { vdom, vapor } = await renderParity(
+      {
+        Child: child,
+        A: wrapper('Child'),
+        B: wrapper('VdomChild'),
+        // an explicit v-bind still merges, like mergeProps
+        C: wrapper('Child', 'v-bind="data.rest"'),
+        App: `<template>
+          <components.A @ready="() => {}" />
+          <components.B @ready="() => {}" />
+          <components.C @ready="() => {}" />
+        </template>`,
+      },
+      () => ref({ style: undefined, rest: {} }),
+      () => {},
+      { VdomChild },
+    )
+    expect(vdom.after).toBe('<i>undefined</i><i>undefined</i><i>object</i>')
+    expect(vapor.after).toBe(vdom.after)
+  })
 })
diff --git a/packages/runtime-vapor/src/component.ts b/packages/runtime-vapor/src/component.ts
index 809f790990f..c7e2f83f325 100644
--- a/packages/runtime-vapor/src/component.ts
+++ b/packages/runtime-vapor/src/component.ts
@@ -88,6 +88,7 @@ import {
   type DynamicPropsSource,
   INITIAL_RAW_VALUES,
   type RawProps,
+  fallthroughOnlyKey,
   getPropsProxyHandlers,
   getStaticBindingKeys,
   hasFallthroughAttrs,
@@ -366,11 +367,15 @@ export function createComponent(
       // must see exactly one fallthrough source
       if (rawProps && rawProps !== EMPTY_OBJ) {
         const sources = (rawProps as RawProps).$
-        rawProps = extend({}, rawProps, {
-          $: sources ? sources.concat(source) : [source],
-        }) as RawProps
+        rawProps = extend(
+          {},
+          rawProps,
+          sources
+            ? { $: sources.concat(source) }
+            : { $: [source], [fallthroughOnlyKey]: true },
+        ) as RawProps
       } else {
-        rawProps = { $: [source] } as RawProps
+        rawProps = { $: [source], [fallthroughOnlyKey]: true } as RawProps
       }
     }
 
diff --git a/packages/runtime-vapor/src/componentProps.ts b/packages/runtime-vapor/src/componentProps.ts
index c4ceada475d..1600ac75613 100644
--- a/packages/runtime-vapor/src/componentProps.ts
+++ b/packages/runtime-vapor/src/componentProps.ts
@@ -49,11 +49,17 @@ import { normalizeEmitsOptions } from './componentEmits'
 import { RenderEffect } from './renderEffect'
 import { interopKey, isInteropEnabled } from './vdomInteropState'
 
+export const fallthroughOnlyKey: unique symbol = Symbol(
+  __DEV__ ? 'fallthrough only' : '',
+)
+
 export type RawProps = Record<string, unknown> & {
   // generated by compiler for :[key]="x" or v-bind="x"
   $?: DynamicPropsSource[]
   // evaluated by vdom, which patches the values in later
   [interopKey]?: boolean
+  // the only dynamic source is the injected fallthrough attrs
+  [fallthroughOnlyKey]?: boolean
 }
 
 export type DynamicPropsSource =
@@ -715,14 +721,21 @@ export function collectProps(
   // Raw attrs may contain prototype-named keys such as __proto__.
   const rawValues: Record<string, any> = Object.create(null)
   const dynamicSources = rawProps.$ || EMPTY_ARR
+  // like vdom, which merges fallthrough attrs only when there are some
+  const attrs = rawProps[fallthroughOnlyKey]
+    ? (dynamicSources[0] as () => Record<string, unknown>)()
+    : undefined
+  const isMerging = attrs
+    ? Object.keys(attrs).length > 0
+    : dynamicSources.length > 0
   for (let i = -1; i < dynamicSources.length; i++) {
     const source = i < 0 ? rawProps : dynamicSources[i]
     const isDynamic = isFunction(source)
-    const resolved = isDynamic ? source() : source
+    const resolved = attrs && i === 0 ? attrs : isDynamic ? source() : source
     for (const key in resolved) {
       if (i < 0 && key === '$') continue
       const value = isDynamic ? resolved[key] : readSource(resolved[key])
-      if (!dynamicSources.length) {
+      if (!isMerging) {
         rawValues[key] = value
       } else if (key === 'class') {
         if (rawValues.class !== value) {
@@ -749,7 +762,7 @@ export function collectProps(
     }
   }
   // merged sources are normalized above
-  if (dynamicSources.length) {
+  if (isMerging) {
     if (rawValues.style) {
       rawValues.style = stabilizeDynamicSourceValue(prevStyle, rawValues.style)
     }