The variant I hear/use here in west coast english goes something like "Oh yeah, no, for sure" for a positive/agreement.

edit: USA west coast English

Yeah nah, that doesn't mean the same as "yeah nah", which means "I acknowledge your reasoning but I do not share your conclusions".

A judge in a court case should be saying "yeah nah" when they come down in favor of the other side; an American who uses many words because it is rude to say "no" is nevertheless just saying "no".