It looks like it's Unicode White_Space, but it misses U+0085 and U+00A0. Some callers account for the latter, but never the former. And many callers don't even realize. Given that isUnicodeWhitespace exists callers could migrate to that instead. simplifyWhiteSpace and stripWhiteSpace that use this underneath will be dealt with in bug 257084.
<rdar://problem/109621392>
Pull request: https://github.com/WebKit/WebKit/pull/14155
Committed 264344@main (c57f446617de): <https://commits.webkit.org/264344@main> Reviewed commits have been landed. Closing PR #14155 and removing active labels.