UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.87 Safari/537.36

Steps to reproduce the problem:
1. Load test case at https://burningchrome.com/~fnd/_/736c2586-8642-4c08-af95-ac22f3051d7f.html#s2 (same as attached; note the fragment identifier).
2. Observe that the heading element's top border is hidden beneath Chrome's chrome.
3. Change the test case to remove `overflow: hidden` and reload (hard).
4. Observe that the heading element's top border is visible now.

In both Firefox 60.0.2 and Safari 11.1.1, the top border is visible irrespective of `overflow`.

What is the expected behavior?
According to #whatwg on FreeNode, this behavior is undefined per spec.

It would be nice if it was consistent across browsers. However, Chrome appears to be the deviant in this case.

What went wrong?
It appears that Chrome uses content-box to determine vertical scroll position with `overflow: auto`, yet border-box otherwise. This seems inconsistent, also when compared with other browsers.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 67.0.3396.87  Channel: stable
OS Version: OS X 10.13.5
Flash Version: 

IRC discussion at https://freenode.logbot.info/whatwg/20180621#c1595824 ff.
 

Deleted: testcase.html 573 bytes

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