:up:, though being broad-minded isn't always the same as not losing your cool (although the two tend to go together). If the main characteristic is 'not losing their cool', I'd use something like unflappable, level-headed or equable (depending on register). 'So laid back s/he's nearly horizontal' is a current idiom.
:up: ....Or 'tolerant' (for the 'forgiving' sense). In some contexts you could also use laissez-faire - which literally means (Freench) 'allow [people] to do [what they want]'; it's chiefly used in politics, in phrases like '...pursued a laissez-faire policy...'
"Tolerance" as used by many today means being absolutely accepting of any behavior of any person, with no judgment that one way of life is any better than another. (For example, homosexuality or single motherhood.)
Those is favor of such "tolerance" are themselves often intolerant with people who hold different views. This is a statement of that. They will not tolerate people who are not as tolerant as they are.