Differentiate NO_FCP failure cases

I’m encountering an issue when running Lighthouse from DevTools: for many performance metrics I get an audit error “No usable firstMeaningfulPaint(Candidate) events found in trace”. However there’s really not too much going on the page, so I’m not sure why it is happening.

However when I run Lighthouse as a Chrome Extension instead I don’t get these errors. I’ve included the JSON export for Lighthouse in DevTools and then as an Extension below.

If there’s any other information you’d like, just let me know!

Lighthouse DevTools JSON export Lighthouse Extension JSON export

[bencollinsuk]

This has been a consistent problem for me and I am convinced it is caused by a frameset. The Chrome extension does not have the problem.

[AttilaVM]

I experienced the same error when I tried to append a WebP image into the DOM from JS, then directly with HTML. Surprisingly the problem vanished when I fell back to PNG.

The WebP image however is rendered perfectly.

So I made very simple HTML file with the same WebP image in its body, it did not trigger the error.

I began to make lesions on my original code. First I removed all the JS code, but the error remained, then I removed the stylus (CSS transpileable language) code, and the error disappeared, I continued, and narrowed down to a simple class with one property, which defines a font family from a custom font face.

The relevant code snippet in the transpiled CSS:

 @font-face {
  font-family: "poppinsRegular";
  src: url("./static/Poppins-Regular.eot");
  src: url("./static/Poppins-Regular.eot?#iefix") format("embedded-opentype"), url("./static/Poppins-Regular.otf") format("opentype"), url("./static/Poppins-Regular.svg") format("svg"), url("./static/Poppins-Regular.ttf") format("truetype"), url("./static/Poppins-Regular.woff") format("woff"), url("./static/Poppins-Regular.woff2") format("woff2");
  font-weight: normal;
  font-style: normal;
}
.primaryFont {
  font-family: "poppinsRegular";
}

Then I changed the font-family to our in-house improved version of Software Tester 7, then to Arial, and in both cases Lighthouse did not throw a NO_FMP error.

So the scary conclusion:

If I use WebP image format and certain type of fonts (presumably not Poppins Regular is the only problematic font) then I get No First Meaningful Paint error. It would be possible to investigate this further, but the time is 1:13 AM here and this is one of the nastiest heisenbugs I’ve ever seen. So I rather go to sleep.

Tomorrow I will try to make a self contained example of this issue.

My environment: OS: NixOS 17.09 “Hummingbird” (Linux 4.9.85) Node: 9.6.1 Chromium: 65.0.3325.162 Lighthouse: 2.9.4