STR:

1. Load attached testcase.

EXPECTED RESULTS:
The two black boxes should be aligned at the same vertical position. (The first one has contain:layout which means it should be treated as having no baseline, just like the second empty box which is actually empty.)

ACTUAL RESULTS:
The first black box uses its contents to establish its baseline, which results in a different baseline position from the second box.

Note: Chrome is not a perfect reference for our expected behavior here -- they do align empty vs. contained boxes consistently, but they do so at the wrong position, per https://bugs.chromium.org/p/chromium/issues/detail?id=946669

I'm fixing this in bug 1552287. I confirmed that the attached testcase has EXPECTED RESULTS in a build with bug 1552287's patches applied.
--> duping forward.