test_same_stream_in_two_tabs_receives_identical_token_sequence started both
SSE-handler threads then immediately put_nowait'd the tokens + stream_end
without waiting for both threads to subscribe. StreamChannel.put_nowait()
buffers events only while there are zero subscribers and clears that buffer
the moment the first subscriber attaches and broadcasts live — so producing
events in the window after tab A subscribes but before tab B does orphans
tab B (subscribes to a cleared buffer, never sees stream_end, hangs ->
thread.join timeout -> 'every tab should finish' fails). ~20% timing flake
on slow CI runners (3.11 shard-4).
Wait until len(stream._subscribers) == len(handlers) (bounded 5s) before
producing events, so delivery is deterministic regardless of thread-start
timing. Test-only; no production change (buffer-then-broadcast is correct for
the real reconnect case).
Verified: original flakes 2/30 under CPU contention; fixed 30/30.
Fixes#5628
Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>
