Currently, if fetch's signal already gets aborted, fetch should reject with the signal's reason if it's not undefined. Related WPT test at https://github.com/web-platform-tests/wpt/blob/83976e7d1f6a7b175947209736bd40e94a7cae77/fetch/api/abort/general.any.js#L37-L45
I will send a patch to fix this later.
Pull request: https://github.com/WebKit/WebKit/pull/15039
<rdar://problem/111201381>
Committed 267033@main (718ac9d81403): <https://commits.webkit.org/267033@main> Reviewed commits have been landed. Closing PR #15039 and removing active labels.