fix: only send response for handled events (fix #9379)

Avoid infinite response event loop by only sending
response messages for events that were successfully handled.

Changes:
- Add eventHandled flag to track successful event processing
- Only call channel.postMessage for handled events
- Add reproduction test

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: simshanith

packages/browser/src/client/tester/tester.ts

@@ -55,6 +55,8 @@ channel.addEventListener('message', async (e) => {
     return
   }
 
+  let eventHandled = true
+
   switch (data.event) {
     case 'execute': {
       const { method, files, context } = data
@@ -96,15 +98,18 @@ channel.addEventListener('message', async (e) => {
       break
     }
     default: {
+      eventHandled = false
       const error = new Error(`Unknown event: ${(data as any).event}`)
       unhandledError(error, 'Unknown Event')
     }
   }
 
-  channel.postMessage({
-    event: `response:${data.event}`,
-    iframeId: getBrowserState().iframeId!,
-  })
+  if (eventHandled) {
+    channel.postMessage({
+      event: `response:${data.event}`,
+      iframeId: getBrowserState().iframeId!,
+    })
+  }
 })
 
 const url = new URL(location.href)

test/browser/fixtures/response-event-loop/response-guard.test.ts

@@ -0,0 +1,33 @@
+import { expect, test } from 'vitest'
+import { channel } from '@vitest/browser/client'
+
+test('response:prefixed event is not processed by tester', async () => {
+  const url = new URL(location.href)
+  const sessionId = url.searchParams.get('sessionId')!
+  const iframeId = url.searchParams.get('iframeId')!
+
+  const secondChannel = new BroadcastChannel(`vitest:${sessionId}`)
+
+  const nestedResponses: string[] = []
+  const handler = (e: MessageEvent) => {
+    if (typeof e.data?.event === 'string' && e.data.event.startsWith('response:response:')) {
+      nestedResponses.push(e.data.event)
+    }
+    secondChannel.postMessage({
+      event: `response:${e.data.event}`,
+      iframeId: e.data.iframeId!,
+    })
+  }
+  secondChannel.addEventListener('message', handler)
+  secondChannel.postMessage({
+    event: 'response:test',
+    iframeId,
+  })
+
+  await new Promise(resolve => setTimeout(resolve, 5))
+
+  secondChannel.removeEventListener('message', handler)
+  secondChannel.close()
+
+  expect(nestedResponses).toHaveLength(0)
+})

test/browser/fixtures/response-event-loop/vitest.config.ts

@@ -0,0 +1,14 @@
+import { fileURLToPath } from 'node:url'
+import { defineConfig } from 'vitest/config'
+import { provider, instances } from '../../settings'
+
+export default defineConfig({
+  cacheDir: fileURLToPath(new URL("./node_modules/.vite", import.meta.url)),
+  test: {
+    browser: {
+      enabled: true,
+      provider,
+      instances,
+    },
+  },
+})

test/browser/package.json

@@ -22,6 +22,7 @@
     "test-setup-file": "vitest --root ./fixtures/setup-file",
     "test-snapshots": "vitest --root ./fixtures/update-snapshot",
     "test-broken-iframe": "vitest --root ./fixtures/broken-iframe",
+    "test-response-event-loop": "vitest --root ./fixtures/response-event-loop",
     "coverage": "vitest --coverage.enabled --coverage.provider=istanbul --browser.headless=yes",
     "test:browser:preview": "PROVIDER=preview vitest",
     "test:browser:playwright": "PROVIDER=playwright vitest",

test/browser/specs/response-event-loop.test.ts

@@ -0,0 +1,15 @@
+import { expect, test } from 'vitest'
+import { runBrowserTests } from './utils'
+
+test('unknown events do not cause infinite response event loop', async () => {
+  const { exitCode, testTree } = await runBrowserTests({
+    root: './fixtures/response-event-loop',
+  })
+
+  expect(exitCode).toBe(0)
+  expect(testTree()).toMatchObject({
+    'response-guard.test.ts': {
+      'response:prefixed event is not processed by tester': 'passed',
+    },
+  })
+})