Created attachment 16763
Older core/MonoBook without regression

Attached:

Hmm, appears to be present in the other skins as well.

Alright, so this appearance change is indeed caused by that commit. I believe it is caused by the part that switches from tables to divs ("Stop using tables use divs instead. It's the future!")

I see three main options:

1. Keep this appearance and better communicate it. I agree the communication here was lax, but it's really not that different. I'm not sure, but I think this layout is also more consistent with the general UI direction.

2. Keep it as divs and use display: table-cell or display: table (I think this still might require markup change, and it needs to be checked on mobile, but it's otherwise pretty straightforward). The downside is that it's still tabular layout, and it's not clear to me we actually want tabular layout here.

3. Go back to table elements, but keep the MW UI styling changes.

I recommend #1.

I think #2 would be preferred, but #1 is also reasonable if #2 is not worth the effort.