Sending a message in the timeline makes it "bounce" on screen

App users (cc @ara4n) have reported that a message sent in the EX apps would "bounce" on screen: first appear, then disappear and reappear quickly, after a short amount of time.

https://github.com/matrix-org/matrix-rust-sdk/pull/6397 shows this behavior in a test. After sending the event, it is saved in the send queue (making the bouncing behavior a fallout of https://github.com/matrix-org/matrix-rust-sdk/pull/5920). The event timeline item state moves from not-sent-yet to sent, with the same TimelineUniqueId. After the remote echo has been received from sync, the event is deduplicated in the event cache, so the updates that are triggered are the following:

• delete the duplicate event (Remove vector diff update)
• insert it back at the expected position (PushBack / PushFront / Insert vector diff update)

Even though the reinserted item reuses the same TimelineUniqueId, it might not be sufficient for embedders to detect that this is a Set in disguise, so the event is first removed from the timeline, and then reinserted.

I think that to properly handle such a case, we'd need some kind of position-aware deduplication mechanism: identify a common head and tail between a list of known events and a list of new events, which would avoid Remove/Insert sequences, so we can turn them into Set instead.

Start with the behavior shown by matrix-rust-sdk#6397 and the send-queue change in matrix-rust-sdk#5920. Trace the event timeline updates, TimelineUniqueId reuse, event-cache deduplication, and resulting Remove/Insert vector diffs; done means the remote echo no longer makes the message visibly disappear and reappear.