Issue with Cumulative Layout Shift metrics in Chrome 86, fixed

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Annie Sullivan

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Oct 29, 2020, 2:42:52 PM10/29/20

to speed-metri...@chromium.org

Hi everyone,

We identified and fixed an issue with the way Chrome 86 calculates Cumulative Layout Shift (CLS). The fix is in Chrome 87, which begins rolling out to the stable channel on Nov 17, 2020. We expect the fix to reach nearly all users by Dec 10, 2020.

In the meantime, CLS data reported from Chrome 86 will be 20% or higher for about 10% of sites.

Mitigation

The easiest mitigation is to ignore CLS data returned by Chrome 86 if you observe elevated CLS values. This will affect data collected roughly between Oct 19 and Dec 10, 2020 when most users were using Chrome 86.

There are no issues with CLS data reported by Chrome 85 and Chrome 87.

Details

Our investigation identified a problem with the way the CLS impact region is calculated in Chrome 86. This issue affects some sites that have elements positioned outside the viewport. The regression was not apparent as Chrome 86 moved through canary, dev and beta channels.

Our findings are published in the metrics changelog and additional details can be found in the tracking bug report.