test(browser): Register the LCP handlers in the bundle of the SDK (#25263)

Fixes a flaky test. The `handlers-lcp` test registered its LCP handlers
in a separate bundle, so they watched LCP through their own observer and
sometimes saw a different element than the SDK did on Linux CI. Now the
handlers are registered in the same bundle as the SDK, so they see
exactly what the SDK sees.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

Author: chargome

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;
+  });
+};

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.