fix(browser): Ensure Standalone CLS span timestamps are correct (#13649)

Fix a bug in the initial experimental CLS standalone span
implementation. Previously we'd add the CLS start timestamp value in ms
to the performance time origin timestamp which was already converted to
seconds. Ensure that we first add time origin and the CLS start timestamp 
and then convert to seconds

---------

Co-authored-by: Abhijeet Prasad <aprasad@sentry.io>

dev-packages/browser-integration-tests/suites/tracing/metrics/web-vitals-cls-standalone-spans/test.ts

@@ -453,3 +453,44 @@ sentryTest("doesn't send further CLS after the first page hide", async ({ getLoc
   // a timeout or something similar.
   await navigationTxnPromise;
 });
+
+sentryTest('CLS span timestamps are set correctly', async ({ getLocalTestPath, page }) => {
+  const url = await getLocalTestPath({ testDir: __dirname });
+
+  const eventData = await getFirstSentryEnvelopeRequest<SentryEvent>(page, url);
+
+  expect(eventData.type).toBe('transaction');
+  expect(eventData.contexts?.trace?.op).toBe('pageload');
+  expect(eventData.timestamp).toBeDefined();
+
+  const pageloadEndTimestamp = eventData.timestamp!;
+
+  const spanEnvelopePromise = getMultipleSentryEnvelopeRequests<SpanEnvelope>(
+    page,
+    1,
+    { envelopeType: 'span' },
+    properFullEnvelopeRequestParser,
+  );
+
+  await triggerAndWaitForLayoutShift(page);
+
+  await hidePage(page);
+
+  const spanEnvelope = (await spanEnvelopePromise)[0];
+  const spanEnvelopeItem = spanEnvelope[1][0][1];
+
+  expect(spanEnvelopeItem.start_timestamp).toBeDefined();
+  expect(spanEnvelopeItem.timestamp).toBeDefined();
+
+  const clsSpanStartTimestamp = spanEnvelopeItem.start_timestamp!;
+  const clsSpanEndTimestamp = spanEnvelopeItem.timestamp!;
+
+  // CLS performance entries have no duration ==> start and end timestamp should be the same
+  expect(clsSpanStartTimestamp).toEqual(clsSpanEndTimestamp);
+
+  // We don't really care that they are very close together but rather about the order of magnitude
+  // Previously, we had a bug where the timestamps would be significantly off (by multiple hours)
+  // so we only ensure that this bug is fixed. 60 seconds should be more than enough.
+  expect(clsSpanStartTimestamp - pageloadEndTimestamp).toBeLessThan(60);
+  expect(clsSpanStartTimestamp).toBeGreaterThan(pageloadEndTimestamp);
+});

packages/browser-utils/src/metrics/cls.ts

@@ -84,8 +84,7 @@ export function trackClsAsStandaloneSpan(): void {
 function sendStandaloneClsSpan(clsValue: number, entry: LayoutShift | undefined, pageloadSpanId: string) {
   DEBUG_BUILD && logger.log(`Sending CLS span (${clsValue})`);
 
-  const startTime = msToSec(browserPerformanceTimeOrigin as number) + (entry?.startTime || 0);
-  const duration = msToSec(entry?.duration || 0);
+  const startTime = msToSec((browserPerformanceTimeOrigin || 0) + (entry?.startTime || 0));
   const routeName = getCurrentScope().getScopeData().transactionName;
 
   const name = entry ? htmlTreeAsString(entry.sources[0]?.node) : 'Layout shift';
@@ -110,7 +109,9 @@ function sendStandaloneClsSpan(clsValue: number, entry: LayoutShift | undefined,
     [SEMANTIC_ATTRIBUTE_SENTRY_MEASUREMENT_VALUE]: clsValue,
   });
 
-  span?.end(startTime + duration);
+  // LayoutShift performance entries always have a duration of 0, so we don't need to add `entry.duration` here
+  // see: https://developer.mozilla.org/en-US/docs/Web/API/PerformanceEntry/duration
+  span?.end(startTime);
 }
 
 function supportsLayoutShift(): boolean {

packages/browser-utils/src/metrics/utils.ts

@@ -86,7 +86,7 @@ export function startStandaloneWebVitalSpan(options: StandaloneWebVitalSpanOptio
   const user = scope.getUser();
   const userDisplay = user !== undefined ? user.email || user.id || user.ip_address : undefined;
 
-  let profileId: string | undefined = undefined;
+  let profileId: string | undefined;
   try {
     // @ts-expect-error skip optional chaining to save bundle size with try catch
     profileId = scope.getScopeData().contexts.profile.profile_id;