Ok sorry if it's not clear. I mean when it happens very suddenly, your almost instinctive reaction with no time to think. The reason I ask is that since my foot can't reach the ground while seated on the saddle unless next to a curb, I would dismount immediately, and so I always feel step-through bikes are much safer. Bikes with high bars scares me. (but please don't turn this thread into a step-through controversy, I'm still interested in your instinctive reaction to avoid an accident)
On a bicycle with a top tube, we usually have clearance so we can straddle the bar.
Most of my bicycles have the top tube, and I can't touch the ground with my foot while I'm seated, so if I have to stop, I remain clipped in with my left foot, slide forward off my saddle, and my right foot plants on the ground. All in one nice neat movement.