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fix(browser): Set artificial zero-value CLS measurement to report no layout shift #12989

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fix(browser): Set artificial zero-value CLS measurement to report no layout shift #12989

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fix(browser): Set artificial zero-value CLS measurement to report no …
Lms24 Jul 19, 2024
0ded265
formatting
Lms24 Jul 19, 2024
9bcc714
cleanup
Lms24 Jul 22, 2024
9b74996
size-limit
Lms24 Jul 22, 2024
94c22d5
Merge branch 'develop' into lms/fix-browser-cls-zero-values
Lms24 Aug 14, 2024
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11 changes: 11 additions & 0 deletions ...ser-integration-tests/suites/tracing/metrics/web-vitals-cls-no-layout-shift/template.html
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Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
</head>
<body>
<div id="content">
Some content
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@Lms24 Lms24 Jul 19, 2024 •
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There needs to be something visible on the page because we (rightfully) only send CLS measurements if FCP was also recorded.

</div>
</body>
</html>
41 changes: 41 additions & 0 deletions ...s/browser-integration-tests/suites/tracing/metrics/web-vitals-cls-no-layout-shift/test.ts
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@Lms24 Lms24 Jul 22, 2024 •
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Note: We have 3 more integration tests covering >0 CLS values which all still pass. So I don't think that this change somehow causes actual layout shift values to be ignored.

Original file line number Diff line number Diff line change
@@ -0,0 +1,41 @@
import { expect } from '@playwright/test';
import type { Event } from '@sentry/types';

import { sentryTest } from '../../../../utils/fixtures';
import { getFirstSentryEnvelopeRequest, shouldSkipTracingTest } from '../../../../utils/helpers';

sentryTest.beforeEach(async ({ page }) => {
await page.setViewportSize({ width: 800, height: 1200 });
});

sentryTest('captures 0 CLS if the browser supports reporting CLS', async ({ getLocalTestPath, page, browserName }) => {
if (shouldSkipTracingTest() || browserName !== 'chromium') {
sentryTest.skip();
}

const url = await getLocalTestPath({ testDir: __dirname });
const transactionEvent = await getFirstSentryEnvelopeRequest<Event>(page, url);

expect(transactionEvent.measurements).toBeDefined();
expect(transactionEvent.measurements?.cls?.value).toBe(0);

// but no source entry (no source if there is no layout shift)
expect(transactionEvent.contexts?.trace?.data?.['cls.source.1']).toBeUndefined();
});

sentryTest(
"doesn't capture 0 CLS if the browser doesn't support reporting CLS",
async ({ getLocalTestPath, page, browserName }) => {
if (shouldSkipTracingTest() || browserName === 'chromium') {
sentryTest.skip();
}

const url = await getLocalTestPath({ testDir: __dirname });
const transactionEvent = await getFirstSentryEnvelopeRequest<Event>(page, `${url}#no-cls`);

expect(transactionEvent.measurements).toBeDefined();
expect(transactionEvent.measurements?.cls).toBeUndefined();

expect(transactionEvent.contexts?.trace?.data?.['cls.source.1']).toBeUndefined();
},
);
28 changes: 28 additions & 0 deletions packages/browser-utils/src/metrics/browserMetrics.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -215,6 +215,8 @@ export { startTrackingINP, registerInpInteractionListener } from './inp';

/** Starts tracking the Cumulative Layout Shift on the current page. */
function _trackCLS(): () => void {
trySetZeroClsValue();

return addClsInstrumentationHandler(({ metric }) => {
const entry = metric.entries[metric.entries.length - 1];
if (!entry) {
Expand All @@ -227,6 +229,32 @@ function _trackCLS(): () => void {
}, true);
}

/**
* Why does this function exist? A very good question!
*
* The `PerformanceObserver` emits `LayoutShift` entries whenever a layout shift occurs.
* If none occurs (which is great!), the observer will never emit any entries. Makes sense so far!
*
* This is problematic for the Sentry product though. We can't differentiate between a CLS of 0 and not having received
* CLS data at all. So in both cases, we'd show users that the CLS score simply is not available. When in fact, it can
* be 0, which is a very good score. This function is a workaround to emit a CLS of 0 right at the start of
* listening to CLS events. This way, we can differentiate between a CLS of 0 and no CLS at all. If a layout shift
* occurs later, the real CLS value will be emitted and the 0 value will be ignored.
* We also only send this artificial 0 value if the browser supports reporting the `layout-shift` entry type.
*/
function trySetZeroClsValue(): void {
try {
const canReportLayoutShift = PerformanceObserver.supportedEntryTypes.includes('layout-shift');
if (canReportLayoutShift) {
DEBUG_BUILD && logger.log('[Measurements] Adding CLS 0');
_measurements['cls'] = { value: 0, unit: '' };
// TODO: Do we have to set _clsEntry here as well? If so, what attribution should we give it?
}
} catch {
// catching and ignoring access errors for bundle size reduction
}
}

/** Starts tracking the Largest Contentful Paint on the current page. */
function _trackLCP(): () => void {
return addLcpInstrumentationHandler(({ metric }) => {
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