Test/dark mode brightness assertions#2097
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dscho merged 2 commits intogit:gh-pagesfrom Oct 2, 2025
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Replace pixel-based screenshot comparisons with perceptually accurate brightness assertions using ITU-R BT.709 relative luminance (0.2126R + 0.7152G + 0.0722B), normalized to [0,1]. Assert light mode ≈0.85 and dark mode ≈0.25. Signed-off-by: rafaeljohn9 <rafaeljohb@gmail.com>
Signed-off-by: rafaeljohn9 <rafaeljohb@gmail.com>
dscho
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Very nice!
The only addition (which I will make in the merge commit's message) is to talk in the commit message about what motivated this change: The PR build failure in #2088 that shouldn't have included a test failure (pointing out that the assertions were ill-specified).
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Changes
getPageBrightness()helper that computes relative luminance using the ITU-R BT.709/sRGB coefficients (0.2126*R + 0.7152*G + 0.0722*B), normalized to the[0, 1]range.0.85 ± 0.1and dark mode is0.25 ± 0.1.Context
Pixel-diff-based screenshot tests are fragile they fail on minor layout shifts, anti-aliasing changes, or browser rendering differences, even when the visual theme (light/dark) is correct.
Instead, we now verify perceived brightness using the standard definition of relative luminance from color science (per Wikipedia), which aligns with human vision sensitivity (green contributes most, blue least).
Reference: #2088 (comment)