Can't believe there's no test case in their automated test suite which would've caught this before being released to tens of millions of users.
Not sure how they test, but they can build a library of typical HTML and CSS edge cases and generate renderings from both an older version and the new version, compare the renderings and flag any discrepancies for manual inspection.
The WebKit LayoutTests is huge and almost certainly covers this. Maybe a bad rebaselining of rendering results, or maybe this is the correct behavior and other WebKit ports will start exhibiting it...
Not sure how they test, but they can build a library of typical HTML and CSS edge cases and generate renderings from both an older version and the new version, compare the renderings and flag any discrepancies for manual inspection.