flushSync() previously deleted pending entries unconditionally
once a transport accepted the payload. That dropped data when
the page was restored from bfcache after visibilitychange:hidden,
and it allowed concurrent flushes to take overlapping batches.
Now items are only removed from `pending` once the transport
confirms delivery: synchronously for sendBeacon, in the keepalive
fetch's `.then` on an `ok` response. Failed fetches leave entries
queued so the next flush can retry, and a `flushing` guard prevents
overlapping in-flight batches.
Signed-off-by: Émile Ré <emile@getprobo.com>
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