Yet what - the meaning in context

Question

She sometimes felt uncomfortably observed, as if living under the unblinking eye of a security camera. Florence wasn’t sure what she’d want to hide from her own son. Yet what best protected privacy wasn’t concealment but apathy—the fact that other people simply weren’t interested.

Am not sure if I'm parsing this right. My guess—there is an ellipsis: Yet what (she was sure about was the fact that) best protected privacy...and so on. But if so, then shouldn't it be Yet that, not Yet what? What here means "the thing which" and it doesn't make sense if we do such a swap in the original sentence: Yet the thing which best protected privacy...??? But it is OK if we use that, not what: Yet (she was sure) that best protected privacy... What do you think of this?

Answer 1

What here means "the thing which" and it doesn't make sense if we do such a swap in the original sentence: Yet the thing which best protected privacy...

It does make sense; that is an understandable sentence.

I suspect that the problem here might be the "yet". In this sentence it functions the same way that "however" or "nevertheless" or "but"; it is used to indicate that the idea "the thing that best protects the privacy of the observed is the apathy of the observer" is partially contradictory to the previously expressed idea: "Florence is uncomfortable being watched and has confusion regarding hiding something from her son".

Try replacing the "yet what" with "however, the thing which" and see if the sentence parses more easily:

However, the thing which best protected privacy wasn't concealment but apathy.

Answer 2

I think that the word "what" is used in sense 2a, "the thing or things that": The thing that best protected privacy was apathy. I don't know why you feel that this does not make sense in the original sentence.

I would explain the word "what" in your paraphrase (What she was sure about was the fact that best protected privacy) in the same way. But I'm not sure what you mean by that paraphrase. Could you paraphrase the complete thought, including the references to concealment and apathy?