> He ended up grabbing me by the throat, while no-one around did a thing to stop him.
The bystander effect is real, but you should also take this opportunity for self-reflection, because in this case, you were the person behaving badly who instigated the situation.
> But I doubt I’ll learn anything from it
Yes, unfortunately it seems unlikely you will.
I’m afraid to say, that if you want a boundary, go home. Otherwise, accept that you’re in public, and people can and will speak to you.
Also, you’ve just justified being violent in response to someone making sounds with their mouth. I bet you’re a calm person to be around, when everyone does what you want.
> In what world do you live?
In what bubble do you live? Go out into the world and behave like the GP. Your apparent mental model of society will collapse quickly.