I'm not discounting those of you who say that someone's leaning over would bother you - it may actually bother you to that extent, and that's a personal thing that no one else can tell you is right or wrong.
But I wouldn't be "mortified" if someone leaned over while showing me something on the computer. Heck, it happens all the time to me. Someone comes by and wants to show me something and leans over to show me on the screen. And I'm sure I've done the same thing. I don't pull back from them like it sounds like this woman did - and they haven't done that to me. I love my personal space as much as the next person. But to be mortified by it seems extreme in just my own experience.
Mortified if they apologized for stinking and launching into the story of how they didn't have deoderant. I am not that nuts about my personal space.