Cumulative Layout Shift Changes in Chrome 91

Cumulative Layout Shift is now computed over a maximum session window

The change is described in more detail in the following series of blog posts:

• Feedback wanted: The road to a better layout shift metric for long-lived pages
• Evolving the CLS metric

There was no change to the trace events or PerformanceObserver API in Chrome, which report individual layout shift events. But the way the layout shifts are accumulated into a Cumulative Layout Shift score was updated for field tools like the Chrome User Experience Report and Google Search Console.

Since these tools pick up data over the course of a month, the release date for the change is closest to Chrome 91, although the changes were made to the Chrome source code earlier. Source code for this change.

These tools started showing the change on June 1.

When were users affected?

Chrome 91 was released on the week of May 31, 2021.