She is keeping her cool instead of registering a reaction to everything. She might well be seething inside, but she keeps herself under control, and in this case it makes for a remarkable contrast. She also never forgets that she’s on camera.
If she can maintain a calm demeanor through all the provocation that her opponent has been offering her, she’s doing well, in my opinion. I keep asking myself as I watch them, “Who’s going to fare better when it comes to the challenges of international diplomacy?” I doubt that anyone she’d meet on the world political scene would ever behave as boorishly and uncivilly in a face-to-face meeting as Trump has in their several appearances together. I’m not a big Hillary fan myself, but I think she has handled herself exceedingly well.