What does "subject to the jurisdiction" "obviously" mean, keeping in mind that everyone agrees children of diplomats aren't citizens at birth, but U.S. courts have jurisdiction over diplomats for some activities?

In general countries have jurisdiction over anyone inside their territory unless there is some exception.

As far as I know the only exceptions at the time the 14th was drafted and ratified would have been people with diplomatic immunity or similar due to treaties and international agreements.

Immediate families of diplomats living with the diplomat are included in diplomatic immunity, hence their children born here would not become citizens.

Those situations you mention where US court do have jurisdiction over diplomats are: private real estate disputes; wills and inheritance; business activity of diplomats that are running a side business or practicing a profession in the US that is not part of their official duties; lawsuits initiated by the diplomat.

Even if becoming subject to such limited jurisdiction counted as being "subject to the jurisdiction" for purposes of the 14th Amendment it would not matter because newborns are not involved in those things, and so newborn children of diplomats have their full diplomatic immunity.

The text that SCOTUS put out today discusses this in great detail. You might agree or disagree with their reasoning, but they consider what "obvious" means in light of the text and the writings of the time plus references to old commentary like Blackstones. Maybe better to read that and assume in good faith that this is what the commenter is referring to?

> […] children of diplomats aren't citizens at birth, but U.S. courts have jurisdiction over diplomats for some activities?

AIUI, IANAL, US courts and law do not have jurisdiction over diplomats. When a diplomat the 'hosting' government agrees to immunity, so cannot be prosecuted. E.g.:

> The collision caused diplomatic tension between British and US officials. [Anne] Sacoolas fled Britain soon after the incident, and claimed diplomatic immunity with US support.

* https://en.wikipedia.org/wiki/Death_of_Harry_Dunn

The 'source' country has to agree to remove the diplomatic coverage.

Well we know “subject to the jurisdiction” is not equivalent to “is a citizen”, otherwise they would’ve used that term instead of being cute with it and leaving excess room for interpretation. If congress wants to define what “subject to jurisdiction” means, then they have every opportunity to.

Have you read the congressional debate around the that amendment to the Amendment? If you have and it's not clear, I'm not sure what else can be said.

Given that the most universally agreed to counter examples are diplomats and invading armies, I'd say a reasonable non-lawyer interpretation is that "subject to the jurisdiction" excludes people who are in some way under the authority or control of a foreign government even while in the US. They may be physically here, but they're presence is on behalf of a foreign government that has jurisdiction over them even while in the US. US law might apply to them in varying ways depending on their status, but there is still a distinction that does not exist for other visitors. Maybe less about the question of whether US law applies at all and more about if US law applies equally beyond the 14th amendment and if any other government can lay claim to authority.

You could potentially apply this to temporary tourists as well, but the linkage between them and the government of the country they are coming from is much weaker since their presence typically doesn't have them acting on behalf of the foreign government or with any special legal distinction.

I'd also pose the same question back to you, what's a reasonable definition of "subject to the jurisdiction" that excludes everyone except US citizens, as many conservatives want, or at least only extends to legal permanent residents?

That's partial jurisdiction at best.

I guess the obvious answer would be the same as the last decades, while anything else would be called "surprising."

Which is is fine, you can change the constitution, but thats for parliament to do.