Quatre gifts him one of the glasses, with a half-smirk and tinge of overdone formality.

There is a moment of hesitance, when he picks a book off of his bed, where he realizes he doesn't know what side Trowa prefers. (Quatre invariably, no matter where he starts, ends up somewhere in the middle; so he doesn't have much preference.) In Milliways Trowa is usually reading, and the lighting is one-sided enough to allow that to make the decision.

He decides to go with that, anyway, and moves to the left. Communication would be awkward, and he's tired.