Regex does not match isolated combining mark as whitespace if preceded by whitespace

Description

I believe Regexes should function on Unicode scalars, not on Swift Chars. This is a failure mode: <space>+<combining mark> (such as " ̃") is seen as a single whitespace character, where all other programming languages I know of regard it conceptually as a single whitespace character plus a single non-spacing combining character.

Reproduction

let aTilde = "ã" // \u{0061} + \u{0303}
let aMatch = try! /\S/.firstMatch(in: aTilde) 
print(aMatch?.output) // "ã" hm... I would have expected only the scalar 'a' to match
let combiningTilde = "̃" // \u{0303}
let tildeMatch = try! /\S/.firstMatch(in: combiningTilde)
print(tildeMatch?.output) // "̃" correct to me
let spaceWithTilde = " ̃" // space+tilde
let spaceTildeMatch = try! /\S/.firstMatch(in: spaceWithTilde)
print(spaceTildeMatch?.output) // nil, but I would expect \u{0303} to match

Expected behavior

tilde scalar was expected to match regex, since it is not a whitespace codepoint (WS) according to Unicode specification, but non-spacing (Mn)

Environment

5.9

Additional information

No response

Start by running the provided Swift reproduction with the three strings and inspect the regex matching path that handles Swift Chars versus Unicode scalars. Done means \S matches only the non-whitespace combining scalar in the space-plus-tilde case while preserving the other demonstrated behavior.