to-device messages can be processed out-of-order, causing dropped keys and decryption errors

[richvdh]

Events - including to-device messages - received over /sync are decrypted in parallel. libolm can handle some reordering of to-device messages: it remembers up to 40 "skipped keys", but in extreme cases, this parallelisation can cause reordering so bad that message keys are forgotten.

[richvdh]

related to https://github.com/vector-im/element-meta/issues/762

[BillCarsonFr]

Adding a test to the complement crypto test list

[kegsay]

Test written and passing, closing.