wpt.fyi: https://wpt.fyi/results/css/css-grid/subgrid/abs-pos-002.html

wpt.live:
test: http://wpt.live/css/css-grid/subgrid/abs-pos-002.html
reference: http://wpt.live/css/css-grid/subgrid/abs-pos-002-ref.html

We have the gray area extending too far to the right on the second line, and too far to the left on the final line.

The discrepancy has to do with the padding on the subgrid element, it seems, and whether that reduces the size of the containing block for the abspos child. The test expects that it does reduce the size of the containing block; but in Firefox, it does not.

Reduced testcase attached, which renders with a skinny gray area in WebKit (as the WPT test is expecting) vs. a wide gray area in Firefox.

It looks like the reference case here changed upstream in this commit:
https://github.com/web-platform-tests/wpt/commit/84f44a7e97041e4e044eaa7e2c51586b716af01b

Discussion in this github PR:
https://github.com/web-platform-tests/wpt/pull/32611