If Stiles had been paying more attention, that smile--real as it is--might be more reassuring, but he's still distracted by the mechanics of what they're about to do.
His fingers lace with Derek's when Derek reaches for him, and he uses it to steady himself as he contemplates where to sit and then just decides--
fuck it--
and straddles Derek's lap, knees on either side of his hips, sinking into the cushions as he settles on top. The angle leaves him slightly elevated, looking down at Derek, and he settles his free hand on Derek's chest, resting over his heart, feeling his heart beat beneath his palm.
"This is us, right?" he asks, quiet, nervous. "This is us," he repeats, forcefully. Not them, he doesn't say.