Can also reproduce on linux, and firefox can render it correctly. I tried bisecting but even the earliest version still reproduces this bug. Removing either z-index or animation makes the bug go away.

Background: transparent on fieldset tag will also make the bug go away.

Note: I checked as far back as 53.0.2767.0 and this bug appears that far back. So doesn't appear to be a regression.

looking at this (no idea how to re-assigned to myself)

put a potential fix here:
https://chromium-review.googlesource.com/c/chromium/src/+/1379169/1

not really an animation bug, will email rossen/skobes to see how to re-assign this bug and who the appropriate reviewers are

Thanks for looking into this rickj@! I believe being able to own bugs requires bug-edit privileges (https://www.chromium.org/getting-involved/get-bug-editing-privileges). I will send an email to committers@ about getting you added.

I sent your CL to the trybots for you; the presubmit will fail because you haven't signed the CLA yet, but the other bots should run and test the CL. Since the CL is in Layout, changing bug component and cc-ing tkent@ to find the appropriate reviewer.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5612ec6b2568a2b4da5444875a9fc443f1867dfd

commit 5612ec6b2568a2b4da5444875a9fc443f1867dfd
Author: Rick James <rickj@microsoft.com>
Date: Tue Dec 18 23:01:31 2018

Fixed background painting of fieldset element with a legend

The BackgroundIsKnownToBeOpaqueInRect return value contract is to return true
when there is full opaque coverage. Field set, with a legend, does not have
full coverage. Need to return false in that case.

This was leading to losing parent painting, most obvious with uninitialized
surfaces of layered parents.

Bug:  892582 
Change-Id: I8c162910d16d6d44301110146739ad1cbf9c88cd
Reviewed-on: https://chromium-review.googlesource.com/c/1379169
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617662}
[modify] https://crrev.com/5612ec6b2568a2b4da5444875a9fc443f1867dfd/AUTHORS
[modify] https://crrev.com/5612ec6b2568a2b4da5444875a9fc443f1867dfd/third_party/blink/renderer/core/layout/layout_fieldset.cc
[modify] https://crrev.com/5612ec6b2568a2b4da5444875a9fc443f1867dfd/third_party/blink/renderer/core/layout/layout_fieldset.h
[modify] https://crrev.com/5612ec6b2568a2b4da5444875a9fc443f1867dfd/third_party/blink/web_tests/TestExpectations

Should be fixed now. Thank you everyone for being so helpful in getting my first Chromium fix completed! :-)