ScriptedSpeaker mutated `handles` AFTER incrementing `_startCount`,
so a reader that polled startCount on a different thread (the test
runs on the runBlocking thread while the broadcast pump runs on
Dispatchers.Default) could see startCount==1 via the volatile
AtomicInteger read before the subsequent list write became visible,
then fail `assertEquals(1, first.handles.size)` with a stale 0.
Also `mutableListOf` itself is not thread-safe — concurrent reads
during a write are undefined.
Reorder so the list append happens BEFORE the increment (the
volatile write now publishes the list mutation under the same
happens-before edge tests already rely on for startCount), and
swap the backing store for CopyOnWriteArrayList so concurrent
reads of `handles[0]` are well-defined.
Observed as a flake on Ubuntu CI; passes locally.
Claude
dd9a530305