Bug 236155 – PerformanceObserver should work with {type: 'paint', buffered: true }

• Status: RESOLVED
• Resolution: DUPLICATE
• Component: DOM

[Andrew Scheuermann]

** Steps to reproduce: **

1. Navigate to example.com (or any web page) in Safari and run the following code in the console:

```
new PerformanceObserver((list) => {
  console.log('callback invoked');
  list.getEntries().forEach(console.log);
}).observe({ type: 'paint', buffered: true });
```

** Actual Results: **

Nothing is logged and the callback is not invoked.

** Expected Results: **

A PerformancePaintTiming entry should be logged. The paint entry is logged with this other approach...

```
performance.getEntriesByType('paint').forEach(console.log);
```

** Additional Information: **
Related Stack Overflow question -> https://stackoverflow.com/questions/70989089/new-performanceobservercb-observe-type-paint-buffered-true-doesnt-w

[Radar WebKit Bug Importer]

<rdar://problem/88513217>

[Simon Fraser (smfr)]

*** This bug has been marked as a duplicate of bug 78011 ***

[Simon Fraser (smfr)]

Not a dup.

[Noam Rosenthal]

This is already covered in a web platform test: https://wpt.live/paint-timing/fcp-only/fcp-iframe.html

From what I checked it works as expected in safari. I also tested it manually in a fresh page.

Note that the observer needs to be added before the paint happens - in your example steps that's not the case.

Can you post a test page where you add an observer before the paint and the observer is not called?

[Andrew Scheuermann]

To clarify, the "Steps to reproduce" show a PerformanceObserver that is created with {type: 'paint', buffered: true}. The presence of the buffered parameter means that the PerformanceObserver callback should invoke with paint entries that occurred ~before~ the observer is added.

You can reproduce this buffered behavior on Chrome, Firefox, Opera, and Edge by copying in the code at any time after the initial paint occurs.


I've updated the bug description to "PerformanceObserver should work with {type: 'paint', buffered: true }"

https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver/observe#parameters

[Noam Rosenthal]

(In reply to Andrew Scheuermann from comment #5)
> To clarify, the "Steps to reproduce" show a PerformanceObserver that is
> created with {type: 'paint', buffered: true}. The presence of the buffered
> parameter means that the PerformanceObserver callback should invoke with
> paint entries that occurred ~before~ the observer is added.
> 
> You can reproduce this buffered behavior on Chrome, Firefox, Opera, and Edge
> by copying in the code at any time after the initial paint occurs.
> 
> 
> I've updated the bug description to "PerformanceObserver should work with
> {type: 'paint', buffered: true }"
> 
> https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver/
> observe#parameters

Ah. I think this is a dup of https://bugs.webkit.org/show_bug.cgi?id=225305

[Noam Rosenthal]

*** This bug has been marked as a duplicate of bug 225305 ***

[Noam Rosenthal]

I marked it as dup, and found an existing failing WPT for this https://wpt.fyi/results/paint-timing/fcp-only/buffered-flag.window.html?label=experimental&label=master&aligned which would make it easier to reproduce and fix.