At https://pixel.wmcloud.org/reports/codex/index.html you'll always see a few failures for the ProgressBar component that are clearly due to a timing issue. It looks like the reference and test screenshots are taken at different points in the animation, which causes a false failure.

We should explore ways (either in Codex or in Pixel) to fix this, maybe by fixing the animation at a certain point in time, or otherwise remove this test.