fix(browser): treat 0 as an implausible LCP value

LCP is a paint time — 0ms is physically impossible. Accepting it let the
uninitialized default (0) leak through as a spurious span when the only
observed LCP value was filtered out as implausibly high.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: logaretm

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

@@ -21,7 +21,7 @@ import { listenForWebVitalReportEvents, msToSec, startStandaloneWebVitalSpan, su
 export const MAX_PLAUSIBLE_LCP_DURATION = 60_000;
 
 export function isValidLcpMetric(lcpValue: number | undefined): lcpValue is number {
-  return lcpValue != null && lcpValue >= 0 && lcpValue <= MAX_PLAUSIBLE_LCP_DURATION;
+  return lcpValue != null && lcpValue > 0 && lcpValue <= MAX_PLAUSIBLE_LCP_DURATION;
 }
 
 /**

packages/browser-utils/test/metrics/lcp.test.ts

@@ -15,13 +15,14 @@ vi.mock('@sentry/core', async () => {
 
 describe('isValidLcpMetric', () => {
   it('returns true for plausible lcp values', () => {
-    expect(isValidLcpMetric(0)).toBe(true);
+    expect(isValidLcpMetric(1)).toBe(true);
     expect(isValidLcpMetric(2_500)).toBe(true);
     expect(isValidLcpMetric(MAX_PLAUSIBLE_LCP_DURATION)).toBe(true);
   });
 
   it('returns false for implausible lcp values', () => {
     expect(isValidLcpMetric(undefined)).toBe(false);
+    expect(isValidLcpMetric(0)).toBe(false);
     expect(isValidLcpMetric(-1)).toBe(false);
     expect(isValidLcpMetric(MAX_PLAUSIBLE_LCP_DURATION + 1)).toBe(false);
   });

packages/browser-utils/test/metrics/webVitalSpans.test.ts

@@ -1,6 +1,7 @@
 import * as SentryCore from '@sentry/core';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import * as inpModule from '../../src/metrics/inp';
+import { MAX_PLAUSIBLE_LCP_DURATION } from '../../src/metrics/lcp';
 import { _emitWebVitalSpan, _sendClsSpan, _sendInpSpan, _sendLcpSpan } from '../../src/metrics/webVitalSpans';
 
 vi.mock('@sentry/core', async () => {
@@ -262,7 +263,7 @@ describe('_sendLcpSpan', () => {
   });
 
   it('sends a streamed LCP span without entry data', () => {
-    _sendLcpSpan(0, undefined);
+    _sendLcpSpan(250, undefined);
 
     expect(SentryCore.startInactiveSpan).toHaveBeenCalledWith(
       expect.objectContaining({
@@ -271,6 +272,13 @@ describe('_sendLcpSpan', () => {
       }),
     );
   });
+
+  it('drops implausible LCP values', () => {
+    _sendLcpSpan(0, undefined);
+    _sendLcpSpan(MAX_PLAUSIBLE_LCP_DURATION + 1, undefined);
+
+    expect(SentryCore.startInactiveSpan).not.toHaveBeenCalled();
+  });
 });
 
 describe('_sendClsSpan', () => {