Issue summary

PSI (PageSpeed Insights) Lighthouse 13.0.1 returns conflicting values for cumulative-layout-shift audit:

• displayValue and numericValue show a non-zero value (e.g. 0.993)
• details.items[0].newEngineResult.cumulativeLayoutShift shows 0 (the actual correct value)
• newEngineResultDiffered: false (engine confirms results match)

This drops the Performance score by ~20 points despite the page actually having no layout shift.

Steps to reproduce

1. Pick any production page that local Lighthouse 13.3 reports CLS = 0 for
2. Run PSI mobile audit on the same page (via API or UI)
3. Inspect the JSON: lighthouseResult.audits["cumulative-layout-shift"]

Current behavior (PSI Lighthouse 13.0.1)

{
  "id": "cumulative-layout-shift",
  "displayValue": "0.993",
  "numericValue": 0.993,
  "score": 0.16,
  "details": {
    "type": "debugdata",
    "items": [{
      "cumulativeLayoutShiftMainFrame": 0,
      "newEngineResultDiffered": false,
      "newEngineResult": {
        "cumulativeLayoutShift": 0,
        "cumulativeLayoutShiftMainFrame": 0
      }
    }]
  }
}

The new engine reports CLS = 0 (correct, page has no shift), but displayValue/numericValue show 0.993 and the score reflects that incorrect value.

Expected behavior

When newEngineResultDiffered: false, the top-level displayValue and numericValue should agree with newEngineResult.cumulativeLayoutShift. Or: the score calculation should use the new engine result.

Comparison: local Lighthouse vs PSI for the same URL

Local Lighthouse 13.3.0 (--form-factor=mobile --throttling-method=simulate):

CLS displayValue: 0
CLS numericValue: 0
CLS score: 1.0
Performance: 94

PSI Google (Lighthouse 13.0.1, US datacenter):

CLS displayValue: 0.993
CLS numericValue: 0.993
CLS audit details newEngineResult.cumulativeLayoutShift: 0
Performance: 73

Both runs target the same public URL with no changes between them.

Impact

Pages with CLS that is actually 0 are penalized by ~20 Performance points in PSI mobile due to displayValue not reflecting newEngineResult. Real users (CrUX) report no CLS, but PSI lab data tells a different story. This causes:

• False alarms in monitoring/dashboards
• Misleading SEO reports
• Manual workarounds in tooling (read CLS from details.items[0].newEngineResult.cumulativeLayoutShift instead of the documented audit fields)

Suggested fix

Either:

1. Sync displayValue/numericValue with newEngineResult.cumulativeLayoutShift when newEngineResultDiffered: false, OR
2. Update PSI's deployed Lighthouse to 13.3.0+ (which already returns the correct values, verified by local runs)

Environment

• Affected: PSI Web/API running Lighthouse 13.0.1
• Not affected: Local Lighthouse CLI 13.3.0 (same audit returns CLS = 0, score = 1.0 for the same page)