Function await_start 

pub(crate) fn await_start(
    py: Python<'_>,
    slot: &Mutex<Option<JoinHandle<()>>>,
    inner: &Inner,
    spawned: ThreadId,
    what: &str,
) -> PyResult<()>

Startup handshake shared by the capture and playback starts: with the GIL released, waits up to ~2 s for the worker this call spawned (spawned) to publish RUNNING or FAILED. A worker still STARTING when the window closes counts as started — its retry ladder legitimately runs longer than this — and a failure after that is exposed through state / last_error instead. On FAILED, join_failed_start tears down only that thread (identity-checked, sparing a concurrent winner) and the worker’s recorded error is raised as RuntimeError.