Bug 305256 – LCP buffer size seems to be set to 1 instead of 150

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Barry Pollard]

This page loads some text and then a delayed `<img>`

https://www.tunetheweb.com/experiments/lcp/h1-image.html

The LCP is correctly reports LCP candidates as:

```
h1#heading
img
```

These are got from and LCP performance observer - basically this code:

```js
new PerformanceObserver((entryList) => {
  for (const entry of entryList.getEntries()) {
    console.log('LCP candidate:', entry.startTime, entry);
  }
}).observe({type: 'largest-contentful-paint', buffered: true});
```

This page also loads some text and then a delayed `<img>`, but delays starting the performance observer for two seconds (after the image has loaded):

https://www.tunetheweb.com/experiments/lcp/h1-image.html

In this case the LCP candidates only contains the second image:

```
img
```

While the largest element is usually the most important for LCP, it's useful for debugging purposes to understand if LCP changed over time, even if a performance observer was setup intitall.

The registry says LCP should have a buffer size of 150:
https://w3c.github.io/timing-entrytypes-registry/#registry

[Barry Pollard]

Urgh, sorry that second test URL should be:
https://www.tunetheweb.com/experiments/lcp/h1-image-delay.html

[Radar WebKit Bug Importer]

<rdar://problem/168015751>

[Simon Fraser (smfr)]

Sure would be nice if https://www.w3.org/TR/largest-contentful-paint/ referenced https://w3c.github.io/timing-entrytypes-registry/#registry

[Barry Pollard]

Agreed! Raised this: https://github.com/w3c/largest-contentful-paint/issues/157

[Simon Fraser (smfr)]

https://www.tunetheweb.com/experiments/lcp/h1-image.html gives me
```
h1#heading
img
```
in Safari, and STP.

Also, in a standalone test that tries to exercise the buffer limit, Chrome seems to have a buffer size of 1.

Do you have a test that explicitly tests the buffer size?

[Simon Fraser (smfr)]

Created attachment 478082 [details]
Prospective test

Adding a WPT that I think should show that the buffer size is > 1.

[Barry Pollard]

> Do you have a test that explicitly tests the buffer size?

Yes as per comment https://bugs.webkit.org/show_bug.cgi?id=305256#c1 the following test gives different results in Chrome and Safari:

https://www.tunetheweb.com/experiments/lcp/h1-image-delay.html

[Simon Fraser (smfr)]

Got it, sorry I missed that (oh for editable comments)

[Simon Fraser (smfr)]

We need an implementation of https://w3c.github.io/performance-timeline/#dfn-queue-a-performanceentry shared between different performance entry types.

[Simon Fraser (smfr)]

Pull request: https://github.com/WebKit/WebKit/pull/63862

[EWS]

Committed 312393@main (86af78950458): <https://commits.webkit.org/312393@main>

Reviewed commits have been landed. Closing PR #63862 and removing active labels.