From 401692e2726a1410a58719396319f5a57b490c46 Mon Sep 17 00:00:00 2001
From: Charly Gomez <charly.gomez1310@gmail.com>
Date: Mon, 12 Oct 2026 11:55:21 +0200
Subject: [PATCH] test(browser): Register the LCP handlers in the bundle of the
 SDK

The handlers lived in the subject bundle, which got its own copy of the
instrumentation and so an LCP observer of its own. On Linux CI that copy
sometimes only ever saw the body candidate while the SDK's copy saw the
image, so the handler assertions failed on every run of some trees and
timed out in others. In the bundle of the SDK the handlers share its
observer and see exactly what the span reports.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 .../tracing/metrics/handlers-lcp/init.js      | 30 +++++++++++++++++++
 .../tracing/metrics/handlers-lcp/subject.js   | 19 +-----------
 2 files changed, 31 insertions(+), 18 deletions(-)
 create mode 100644 dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/init.js

diff --git a/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/init.js b/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/init.js
new file mode 100644
index 000000000000..05b690d85393
--- /dev/null
+++ b/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/init.js
@@ -0,0 +1,30 @@
+import * as Sentry from '@sentry/browser';
+import { addLcpInstrumentationHandler } from '@sentry/browser-utils';
+
+window.Sentry = Sentry;
+
+Sentry.init({
+  dsn: 'https://public@dsn.ingest.sentry.io/1337',
+  integrations: [Sentry.browserTracingIntegration()],
+  tracesSampleRate: 1,
+});
+
+// Registered in the bundle of the SDK, so the handlers share its LCP observer. A second bundle gets
+// a copy of the instrumentation with an observer of its own, whose view of the LCP candidates can
+// differ from the SDK's (e.g. a different first-hidden time), which made this test flaky.
+addLcpInstrumentationHandler(({ metric }) => {
+  const entry = metric.entries[metric.entries.length - 1];
+  window._LCP = entry.size;
+});
+
+addLcpInstrumentationHandler(({ metric }) => {
+  const entry = metric.entries[metric.entries.length - 1];
+  window._LCP2 = entry.size;
+});
+
+window.ADD_HANDLER = () => {
+  addLcpInstrumentationHandler(({ metric }) => {
+    const entry = metric.entries[metric.entries.length - 1];
+    window._LCP3 = entry.size;
+  });
+};
diff --git a/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/subject.js b/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/subject.js
index 838e6e864414..d57faa5bc8b1 100644
--- a/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/subject.js
+++ b/dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/subject.js
@@ -1,18 +1 @@
-import { addLcpInstrumentationHandler } from '@sentry/browser-utils';
-
-addLcpInstrumentationHandler(({ metric }) => {
-  const entry = metric.entries[metric.entries.length - 1];
-  window._LCP = entry.size;
-});
-
-addLcpInstrumentationHandler(({ metric }) => {
-  const entry = metric.entries[metric.entries.length - 1];
-  window._LCP2 = entry.size;
-});
-
-window.ADD_HANDLER = () => {
-  addLcpInstrumentationHandler(({ metric }) => {
-    const entry = metric.entries[metric.entries.length - 1];
-    window._LCP3 = entry.size;
-  });
-};
+// The handlers live in `init.js`, see there.