From 46a3cf750b51c0f3b0226d0225675ed559882b80 Mon Sep 17 00:00:00 2001
From: Shane Daniel <135388+simshanith@users.noreply.github.com>
Date: Wed, 29 Jul 2026 00:35:15 -0700
Subject: [PATCH] fix(browser): ignore channel events in a tester opened as a
 top-level window (fix #9379) (#9381)

---
 packages/browser/src/client/tester/tester.ts | 18 ++++--
 test/browser/specs/duplicate-tester.test.ts  | 60 ++++++++++++++++++++
 2 files changed, 74 insertions(+), 4 deletions(-)
 create mode 100644 test/browser/specs/duplicate-tester.test.ts

diff --git a/packages/browser/src/client/tester/tester.ts b/packages/browser/src/client/tester/tester.ts
index 7129a15c6..c2790dda3 100644
--- a/packages/browser/src/client/tester/tester.ts
+++ b/packages/browser/src/client/tester/tester.ts
@@ -35,9 +35,17 @@ const traces = new Traces({
 let rootTesterSpan: ReturnType<Traces['startContextSpan']> | undefined
 getBrowserState().traces = traces
 
+// a tester opened as a top-level window (e.g. a test clicking <a target="_blank">
+// at the tester URL) would echo `ack:`/`response:` events back recursively
+const isEmbedded = window.self !== window.top
+
 channel.addEventListener('message', async (e) => {
   const data = e.data
 
+  if (!isEmbedded) {
+    return
+  }
+
   if (!isEvent(data)) {
     await client.waitForConnection()
     const error = new Error(`Unknown message: ${JSON.stringify(e.data)}`)
@@ -140,10 +148,12 @@ getBrowserState().iframeId = iframeId
 
 registerPageMarkHandler((name, options) => page.mark(name, options))
 
-channel.postMessage({
-  event: 'ready',
-  iframeId,
-})
+if (isEmbedded) {
+  channel.postMessage({
+    event: 'ready',
+    iframeId,
+  })
+}
 
 let contextSwitched = false
 
diff --git a/test/browser/specs/duplicate-tester.test.ts b/test/browser/specs/duplicate-tester.test.ts
new file mode 100644
index 000000000..667bd3555
--- /dev/null
+++ b/test/browser/specs/duplicate-tester.test.ts
@@ -0,0 +1,60 @@
+import { expect, test } from 'vitest'
+import { instances, runInlineBrowserTests } from './utils'
+
+// clicking <a href="" target="_blank"> opens the tester URL (query string included)
+// as a top-level window — the duplicate tester must not join the iframe channel
+test('a popup opened at the tester URL does not corrupt the iframe channel', { timeout: 60_000 }, async () => {
+  const { stderr, exitCode, testTree } = await runInlineBrowserTests(
+    {
+      'popup.test.ts': `
+        import { expect, test } from 'vitest'
+        import { userEvent } from 'vitest/browser'
+
+        test('clicking a target="_blank" anchor', async () => {
+          const anchor = document.createElement('a')
+          // href="" resolves to location.href — the tester page itself
+          anchor.setAttribute('href', '')
+          anchor.setAttribute('target', '_blank')
+          anchor.textContent = 'open'
+          document.body.appendChild(anchor)
+
+          // real user gesture so the browser allows the popup
+          await userEvent.click(anchor)
+
+          // give the popup time to boot the duplicate tester
+          await new Promise(resolve => setTimeout(resolve, 1000))
+
+          expect(anchor.target).toBe('_blank')
+        })
+      `,
+      // more execute/cleanup events after the duplicate tester exists
+      'second.test.ts': `
+        import { expect, test } from 'vitest'
+
+        test('a following test file still runs cleanly', () => {
+          expect(1 + 1).toBe(2)
+        })
+      `,
+    },
+    {
+      // fail fast: a corrupted channel loses the real tester's `ack:cleanup`
+      env: { VITEST_BROWSER_IFRAME_TIMEOUT: '10000' },
+      browser: {
+        instances: [instances[0]],
+      },
+    },
+  )
+
+  expect(stderr).not.toContain('Unknown event')
+  expect(exitCode).toBe(0)
+  expect(testTree()).toMatchInlineSnapshot(`
+    {
+      "popup.test.ts": {
+        "clicking a target="_blank" anchor": "passed",
+      },
+      "second.test.ts": {
+        "a following test file still runs cleanly": "passed",
+      },
+    }
+  `)
+})
-- 
2.51.2