# 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](https://web.dev/better-layout-shift-metric/)
* [Evolving the CLS metric](https://web.dev/evolving-cls/)
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](https://developers.google.com/web/tools/chrome-user-experience-report)
and [Google Search Console](https://search.google.com/search-console/about).
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](https://chromium-review.googlesource.com/c/chromium/src/+/2556583).
These tools started showing the change on June 1.
## When were users affected?
Chrome 91 was released on the week of May 31, 2021.
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