The right side of Proton tabs render differently in Software WebRender when compared to both Hardware WebRender and Basic. The behavior is consistent across hardware and OS (example screenshots from macOS, but the difference is the same in Windows 10 on another machine).

Mozregression says this was caused by bug 1702329, which was a CSS change. Since this only looks bad in SW-WR, I am filing against that, but there is a chance this could be theme-related.

We accidentally were clamping the horizontal stage of the separable blur some
number of samples too early before the end of the row, rather than at the end
of the row.

https://hg.mozilla.org/mozilla-central/rev/a1b461b1982e

Comment on attachment 9222313 [details]
Bug 1710325 - Fix offset clamping in SWGL blur fast-path. r?jrmuizel

Beta/Release Uplift Approval Request

• User impact if declined: Visual artifacts in some Proton UI chrome for Software WebRender users (which is being rolled out in 89).
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): The impact is mostly confined to SW-WR users, and it is mostly a small cosmetic change.
• String changes made/needed:

Lee, you patch does not graft to beta cleanly as it changes tests that landed in bug 1706716 in 90, do you want to provide an updated patch for beta? Thanks

I've rebased this version of the patch for 89. It will apply now.

Comment on attachment 9222642 [details] [diff] [review]
Bug 1710325 - Fix offset clamping in SWGL blur fast-path (89). r=jrmuizel

Approved for 89 beta 15, thanks.

https://hg.mozilla.org/releases/mozilla-beta/rev/f6363fa92c42