Description
For code /[^\P{Lowercase_Letter}]/iu.test('A'), according to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class#complement_classes_and_case-insensitive_matching and Web reality (Chrome/Firefox/Safari), it should be false, but true as my understanding of the specification, I'm not sure if I read it correct.
S is a CharSet that includes all code points that do not have the property Lowercase_Letter, and has Simple Case Folding applied, therefore A was originally in the CharSet (it is not a Lowercase_Letter) but after Simple Case Folding applied, it becomes a.
In step 3, it calls CharacterComplement on S, so a does not appear in the returned CharSet but A does. (let's call it S2)
In step 2.k, the input character ch=A and converted to cc=a after canonicalized.
In step 2.l, it searches for a in the CharSet S2 above, and found is false.
invert is true and found is false, so the match succeeds (not the same with MDN and web result).