Is 'And you?' idiomatic? [closed]

Solution 1:

Language use changes according to context. In English, when you are in familiar or relaxed contexts the vocabulary and grammar often become more informal, and more informal sentence structures often results in less words being used to convey the meaning.

If words can be left out, but inferred from the context, then that often happens in informal settings. This is what is happening here. In the most familiar settings, sometimes words can be dispensed with entirely and a glance or raised eyebrow can communicate the same meaning.

The example you give sounds natural to me. I wouldn't call it "idiomatic" since it's not using words differently from their basic meaning in a way particular to English. It's just informal sentence structuring.