The synchronous pre-timeout in the implementation was a band-aid for a
missing flushSync() in the third test. $effect callbacks are scheduled
as microtasks and don't run until flushed — the test set source=true
before the initial effect had a chance to register the setTimeout, so
advanceTimersByTime fired nothing.
Proper fix: call flushSync() after $effect.root so the initial effect
runs synchronously, then advance fake timers. Removed the duplicate
setTimeout branch from useDelayed — one code path, no race.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
bvandeusen
19d74d00d6