What made the window Chucking great(aside from John's horrified expression) is that she just got up without even wondering what happened. (which is very much a Terminator thing, but...)

I think Cameron's cluelessness is something Sarah was counting on, because her silence gave the effect of her being a petulant teen, which is something Sarah counted on. It shows that Sarah was thinking on her feet even more than the obvious, because she was taking what she knew of Cameron's actions and reactions into account and using them to her advantage, not just hoping Cameron would play along.