I don't see it as harmless, although luckily I didn't lose anything as it happened to be a freeroll, but I could have. Worse, it could have happened at a cash table and then I would have lost money.
Minimizing could help, I guess, and I had never thought I needed to. But your comment about locking the screen is interesting, how would one do that? I've never come across that, but it's interesting for more than just to protect my poker funds! I'm not all that technical - oh wait, it sounds like something that would happen on a phone or tablet. I play on my PC - can you do it on there?
Hopefully, like you say, the fact that I wasn't too happy with her anyway is enough to stop it happening again anyway, but the only other thing I can think of is switching off the monitor. Not exactly a concrete method if someone was determined, but maybe enough to stop temptation which I think is what happened here?
Have to say though, it does illustrate that online isn't as secure as the casino - they'd notice if she sat in my seat there!