This seems to manifest as fuzziness in layout/reftests/css-blending/mix-blend-mode-and-filter.svg like it is sampling something in the filter at a wrong offset, once SWGL enabled KHR_advanced_blend_equation support. As per Glenn on matrix:

I'm not entirely surprised, I'm finding it difficult to understand how any of the mix-blend stuff works in the presence of raster roots (which svg filters introduce) at the moment!
(well, it doesn't work - I've already been able to create simple test cases that are completely broken with the existing impl)

I am working on this now, as it needs to be fixed to unblock the mix-blend optimization work I'm doing anyway.

This fixes incorrect rendering when either the source or backdrop
tasks establish a raster root.

By design, it also changes mix-blend backdrop readbacks to work in
a way that can handle readbacks from picture cache tiles, which is
a follow up optimization being worked on.

Backed out changeset ddd89e758a23 (bug 1690396) for Wrench failures. CLOSED TREE

Log:
https://treeherder.mozilla.org/logviewer?job_id=328786345&repo=autoland&lineNumber=13437

Push with failures:
https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&selectedTaskRun=arKd_SyOQ5Gg7udz2K5Sdg.0&revision=ddd89e758a23d130d6967c9ac71676fd540ad011

Backout:
https://hg.mozilla.org/integration/autoland/rev/097886f855f15b0d312bfcd763d4c2cccc2febf7

Oops, I had already found and fixed this failure - but forgotten to push the updated patch to phab. Should be fixed now, thanks!

https://hg.mozilla.org/mozilla-central/rev/0788e77d2d62

Backed out for causing Bug 1690846

backout: https://hg.mozilla.org/mozilla-central/rev/f259675c88bf6bb78eed73238ba942804417c750

Thanks for backing out! Do we have any URLs from the crash reports? I haven't managed to repro locally yet.

https://hg.mozilla.org/mozilla-central/rev/6b775dfe44a9