"It was only a second"

The glance is quick. You'd swear it was a second. Possibly it was.

The trouble is who's timing it. You're timing from the moment you decided to look to the moment you looked back, and that's short. The person talking is timing something different. They're timing from the moment your face changed to the moment it came back, and that's the drop, and it starts before your glance does.

Faces change before eyes move. The tug arrives, and something goes out of your expression, and the person watching you sees it go. Then your eyes go down. Then, on the way back, there's a moment where your eyes are on them but you're still finishing the thought from the screen. They see that too. My reading of what's going on is that the drop runs from the first flicker to the full return, and the glance is the middle third of it.

Varoth Chotpitayasunondh and Karen Douglas tested how much of a conversation partner's phone use it takes to make a difference. They had people watch a short animation in which they imagined themselves in a two-person conversation, and the partner either looked at their phone a lot, some of the time, or not at all. More phone use made the conversation feel worse and the relationship feel worse, and the effect ran through feeling less that they belonged and through worse mood. The partial condition wasn't safe. It was the middle of a slope.

This was an animation, not a dinner, and people imagining a conversation may react differently from people having one. But it points the same way as the restaurant and the museum, and it says something the restaurant couldn't. What matters isn't the total time on the phone. It's the amount of looking away the other person sees, and they see all of it.

So when you say it was only a second, you're right about the part you measured. The person across from you measured the whole thing, and the whole thing is what they'll remember about the evening, if they remember anything.