Standard only state that auto-commit happen after requests finished and results handled.
Results handle by related events so implementation might wait for event queue to be empty.

When a transaction has been started and it can no longer becomeactive, the implementation must attempt to commit it, as long as the transaction has not beenaborted. This usually happens after all requests placed against the transaction have been executed and their returned results handled, and no new requests have been placed against the transaction.

fromindexeddb api 2.0