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From what I've read on CLS it's worked out with the following formula (essentially stuff in the viewport):

  • area of impact region / area of viewport = impact fraction
  • max move distance / viewport height = distance fraction
  • impact fraction x distance fraction = layout shift score for a single animation frame
  1. Does google work out the CLS of content below the fold? It should in theory, as for example if I'm quite far down a page and click on a link, then i want to go back to the previous page.. All of the content above where I was has to not shift so that I got back to the same place on that page. But on Google's Page Speed Insights the screengrab is only of above fold content.

  2. Or is it a case of LAB data working out above the fold CLS and FIELD data working out all of the CLS everyone ever encounters in the last 28 days? including pressing back onto a page (that you've scrolled a lot on) and things shifting.

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    Google doesn't collect data from everyone. They only collect data from chrome users who share their browsing experiences with Google. Google calls the reporting service the Chrome UX report. Users have usage statistics and crash reports enabled to have their data included in it. (This setting used to be opt-in, but as Chrome 54 it enabled by default and users have to opt-out.) Mar 9, 2021 at 14:39

2 Answers 2

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The lighthouse CLS score is calculated based on the current viewport it is being tested in.

For example, if you test your site on a 1366px monitor and then on a 1920px monitor, the scores may be different (depending on your code quality).

So if you're seeing a bad CLS score from the collaborative data on the Page Speed Insights tool, or in the Google search console, it may be that one of your responsive screen sizes has a bad layout shift that is dragging down the overall accumulated score.

You can easily test how the CLS scores differ. Just open lighthouse, do a test with the page scrolled to the top, and then do a test with the page halfway scrolled down, and you'll see the CLS scores are vastly different.

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Evolving the CLS metric

we've finalized the change we plan to make to the metric: maximum session window with 1 second gap, capped at 5 seconds.

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