Possible racing conditions with coediting sync

[pklaschka]

While I haven't experienced this yet, it is possible that editing transactions could override each other, meaning messages that get, say, sent at the same time might interfere or override each other.

Possible (unconfirmed) way to reproduce:

1. A clicks the delete button of a message (this opens the confirmation dialog and begins the scenegraph editing transaction)
2. B sends a message (transaction starts and finishes)
3. A confirms the message deletion (A's transaction finishes)

Also, this might happen with messages getting sent at the same time. The best way to reproduce this would probably be for one person to "spam-send" a burst of messages with another user trying to send a message within such a burst.

[pklaschka]

We might achieve this by storing the last transaction id (a unique ID) of a user in the plugin and the list of the most recent edit transactions in the document itself. Then, when the document updates, the plugin can check whether its last transaction id (which should be there) is still "in the document." If not, a warning with a CTA to reperform the transaction (sending the message or something similar) gets shown.