Issue from element-x-ios: Timeline can get stuck showing only the most recent message if there's a reset which races with a local echo

[Hywan]

Originally written by @ara4n at https://github.com/element-hq/element-x-ios/issues/5817

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Steps to reproduce

1. Send messages at the same time that the timeline is coincidentally getting reset by SSS for whatever reason
2. It then only shows the most recent message (and the date separator)

Here seen on macOS on build 1255 but the same applies on iOS:

There's no loading spinner (although overscrolling the screen kicks it into attempting a backpagination, which fixes it).

Outcome

What did you expect?

The timeline should never get stuck showing partial history.

What happened instead?

So, I stumbled across this while smoketesting the Tachyon DMLS work a few weeks ago via Claude: I fixed it on my experimental branch but haven't had a chance to try to upstream it. So before I forget it again, three possible related issues were thrown up. I don't the code well enough to unpick which of them (maybe all?) fix this failure mode - you can find them at https://gist.github.com/ara4n/d9462cca5c758c5b8f6a9536a1278c85.

• One fixes the actual race between timeline reset & echo:
  • https://gist.github.com/ara4n/d9462cca5c758c5b8f6a9536a1278c85#file-fix-race-between-timeline-reset-and-local-echo-patch
• There's one which stops duplicate messages appearing if timeline backfill races with local echo:
  • https://gist.github.com/ara4n/d9462cca5c758c5b8f6a9536a1278c85#file-fix-timeline-duplication-patch
• Finally, another fixes a similar problem where the timeline gets stuck if /messages ever returns empty:
  • https://gist.github.com/ara4n/d9462cca5c758c5b8f6a9536a1278c85#file-fix-breakage-on-empty-backfill-patch

@Hywan i'm hoping something useful can be salvaged from the claude output here if it'd be possible to take a look. I get the local-echo-race bug once every few days.

I can also provide logs if needed.

Application version

1255

Will you send logs?

No