`useReportWebVitals` (LCP) Intermittently Not Reporting When User is Idle

[samtbg]

Link to the code that reproduces this issue

https://github.com/samtbg/userReportWebVitalsMissingLCP

To Reproduce

1. Clone the linked repository and run the necessary package install command:

npm install --legacy-peer-deps

2. Start the development server:

npm run dev

3. Navigate to http://localhost:3000.

4. Open the browser DevTools Console and clear the logs.

Test 1 (Failure Scenario - Idle User)
This demonstrates the missing LCP report:

Reload the page.

Wait 12 seconds without moving or clicking the mouse.

Result: The console will show the "10-second timer fired" message. The OTel flush runs, and the log will confirm TTFB and FCP were sent, but LCP is missing.

Test 2 (Success Scenario - Interactive User)
This demonstrates the required workaround:

Reload the page.

Wait 5 seconds (the LCP element is visible).

Click once anywhere on the white space. The console should show the LCP metric recorded immediately after the click.

Wait for the 10-second timer to fire (5 more seconds).

Result: The console will successfully show TTFB, FCP, and LCP being sent.

Current vs. Expected behavior

Current Behavior (Observable in the Console):

Test 1 (Failure): The console immediately logs the recording of TTFB and FCP. At approximately 10 seconds, the console logs the "10-second timer fired" and the OTel flush runs. The final flush log shows that LCP is missing from the buffer. The useReportWebVitals callback for LCP never executes.

Test 2 (Success): When a click on the page occurs, LCP is recorded and reported in the console

Expected Behaviour

The LCP is recorded/reported irregardless of user interaction

Provide environment information

From the reproduction:

Operating System:
  Platform: linux
  Arch: x64
  Version: #88-Ubuntu SMP PREEMPT_DYNAMIC Sat Oct 11 09:28:41 UTC 2025
  Available memory (MB): 39737
  Available CPU cores: 8
Binaries:
  Node: 20.11.0
  npm: 11.4.2
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 16.0.2-canary.24 // Latest available version is detected (16.0.2-canary.24).
  eslint-config-next: N/A
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.4.5
Next.js Config:
  output: N/A

However we're seeing this behaviour in our NextJS apps (which have a wide range of versions)

Which area(s) are affected? (Select all that apply)

Performance

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

No response