1841355 - WPT skew-test1.html renders with a single bright red pixel, which should be covered up

Status: NEW

(Core :: Graphics: WebRender, defect)

Description

[Daniel Holbert [:dholbert] (PTO until 2/24)]

Per bug 1841349 (and screenshot attachment 9341994 [details]), our test-runners capture a screenshot of this test showing a single solid-red pixel, well inside the bounds of the green shape, which should clearly be covered up or at least blended:
http://wpt.live/css/css-transforms/skew-test1.html

I'm filing this bug to track this issue, and I'll be adding a known-to-fail reftest over in bug 1841349 to help us detect if this issue happens to become fixed.

Comment 1

[Daniel Holbert [:dholbert] (PTO until 2/24)]

Update: I changed the test substantially to make it more robust and avoid spurious fuzzy-failures, so the up-to-date WPT test linked in comment 0 isn't exercising this anymore.

I landed a reftest with an expected-to-be-quite-fuzzy annotation that does continue to trigger this, though:
https://searchfox.org/mozilla-central/source/layout/reftests/bugs/1841355-1.html

That test is annotated as fuzzy-if(!useDrawSnapshot&&!swgl,254-255,110-121), where the 254-to-255 color-channel-difference (under hardware WebRender) is due to this bug here. That test will help us validate whether this bug remains (since the test will start unexpectedly-passing if we improve); and hopefully someone can use that test as part of investigating tentative fixes here in the future.