STR:

1. Enable about:config pref layout.css.content-visibility.enabled
2. Visit this WPT test:
   https://wpt.live/css/css-overflow/overflow-clip-margin-007.html
3. Compare the rendering to reference-case:
   https://wpt.live/css/css-overflow/overflow-clip-margin-007-ref.html

EXPECTED RESULTS:
Consistent rendering between testcase and reference.

ACTUAL RESULT:
There are two differences:
(A) Testcase renders blank (whereas the reference case has a green strip).
(B) Testcase's scroll position seems different from the reference case (i.e. the reference case has a further overall height)

Difference (A) seems to be an invalidation bug of some sort. It goes away (i.e. we paint the green strip) if you click the testcase or do Ctrl+A to select all.

Difference (B) seems to be a broader difference, where we actually disagree with Chromium and WebKit on the document.documentElement.scrollHeight of the reference case (though we agree with them on the testcase when the content-visibility pref is enabled). That's not directly related to content-visibility, I think, so I'll spin of a dedicated bug for that behavior difference.

(In reply to Daniel Holbert [:dholbert] from comment #0)

Difference (B) seems to be a broader difference [...] I'll spin of a dedicated bug for that behavior difference.

I filed bug 1820266 on that.