Is this cheating

Did she ask for it?? Sounds more like an assault and literally miles away from cheating. Your question is a weird super jump to conclusion sentiment.

Or are you just looking for some super lame excuse to accuse her of something she didn't do and break up with her? Or are you usually this toxically jealous that you'd accuse her of cheating if she had the nerve to share an elevator with a stranger?

Why she didnt move away because this last for some time. It was almost like kissing

How would I (a complete stranger on the internet and someone who literally cannot read your GF's mind) know?

Maybe she was starstruck by the obvious power move? Not sure what kind of genre the music we're talking about here? But singers usually are charismatic and quite alluring. That's still not cheating. Even if the singer forced a literal kiss on her. Not like she went to his dressing room at immediately then or after the concert and shagged the singer? Or did she? Seems to me you're protesting too much and looking for dumb things to drag her down a notch. An excuse to shame her or break up with her. Nonsense talk.

No

You sound unhappy because you are making a big deal out of nothing. She did not cheat, and I'm sure she didn't know what to do in such a situation right in the middle of a spotlight.

Exactly.

Part of the show.

It's not her "cheating", it's your irrational jealousy.