Intolerance in Popper's sense has a different connotation than we might be used to today. The thinking of Karl Popper is heavily influenced by the horrors of the holocaust where people where denied rights, quartered up, shot or gassed based on being in the oposition, having the wrong ideology, religion, skin color, relatives, friends, gender identity, sexuality, etc. So for Popper this was not some vague undecidable "what is intolerance anyways"-kind of word, but bound to really harsh existential inhumane cruelties.

I guess we can agree that people who want others killed or at least surpressed like that do count pretty much as intolerant under regular free society standards. So in other words: intolerant people are people who can't be bothered to tolerate sharing their existance with other people who mind their own bussiness.

So: the definition of the word "intolerance" decides who is intolerant. And of course intolerant people will argue that they aren't (because that is not a nice attribute to get branded with, even if it is true) and therefore they try to change the meaning of "tolerance" in their image. And of course some people will claim intolerance where the definition won't justify it, because they feel it gives them an advantage in a given situation and therefore try to change the definition of "tolerance" into their direction. Ultimately like any codified law it would be a matter of presedence etc.

The ones with the power to decide or power to yield that power to another, as what already happens anywhere you go. You can't murder someone for slighting you, that would be intolerant in US law. Don't like it, yield power to change the law.