| # Changes related to FCP and document opacity in Chrome 102 |
| |
| ## Opacity changes and FCP |
| When investigating an |
| [issue](https://bugs.chromium.org/p/chromium/issues/detail?id=1279548), we |
| discovered that while FCP is properly ignored when the document is fully opaque |
| (e.g. in cases where an A/B anti-flicker snippet is deployed), it does not |
| necessarily fire when the document's opacity changes, unless there are paint |
| involved with that opacity change. One particular case where paints are elided |
| is when a `will-change: opacity` style is applied to the document. |
| |
| ## How does this affect a site's metrics? |
| If your site is using an A/B testing framework that employs |
| [full-document opacity snippets](https://andydavies.me/blog/2020/11/16/the-case-against-anti-flicker-snippets/), |
| then this issue may have affected your site's FCP metrics. |
| |
| The issue results in an FCP entry that indicated the time of the next |
| contentful paint after the opacity change, which could be significantly after |
| the actual FCP. Potentially, there could also be cases where FCP wasn't |
| reported at all. |
| |
| ## When were users affected? |
| This seems to have been a long standing issue, most likely introduced when we |
| [started ignoring paints with opacity 0](https://chromium-review.googlesource.com/c/chromium/src/+/2316788) |
| in Chrome 86. |
