When 'sure' doesn't mean sure

“You and your partner are unpacking groceries together after a Saturday shop — bags on the counter, fridge open, a comfortable rhythm. You mention offhand that your coworker invited you both to their housewarming next weekend, and your partner says 'oh, sure' while putting something away. You catch it before it disappears: not a real 'sure.' More like the word someone uses when they've already decided not to make a thing of it.”