Enable pagination for overlay timelines

[robintown]

Up until now, when events from a virtual room (see internal documentation) are overlaid on the timeline of the native room, only the first page of events would be overlaid. This change enables TimelinePanel to load in as many overlay events as necessary to cover whatever section of the native room's timeline the user happens to be looking at, and also accounts for the timeline window moving around as the user scrolls through history.

Closes https://github.com/vector-im/voip-internal/issues/107

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Here's what your changelog entry will look like:

✨ Features

• Enable pagination for overlay timelines (#10757). Fixes vector-im/voip-internal#107.

[t3chguy]

This approach isn't really right for aligning timelines - matrix-org/matrix-js-sdk#3325

localTimestamp is based on age which can even be negative

The time in milliseconds that has elapsed since the event was sent. This field is generated by the local homeserver, and may be incorrect if the local time on at least one of the two servers is out of sync, which can cause the age to either be negative or greater than it actually is.

[robintown]

Unfortunately we do need to use a timestamp-based approach to event ordering here, since the events are from entirely different rooms, and as such the server doesn't give us any hints as to what order it received them in.

My understanding is that localTimestamp is the most consistent measure we have for creating an order relative to the local clock. Or would it be better to use origin_server_ts instead here?

[t3chguy]

I think this warrants a spec discussion on how this is meant to be done

[robintown]

I've raised this as a problem needing further consideration in the spec repo: matrix-org/matrix-spec#1520

For now though, I would note that the localTimestamp approach to aligning overlay events is nothing new introduced by this PR, it's only been refactored here. My intent in extracting the comparison into these two functions, was so that we could easily switch to a better approach if one eventually presents itself. There's also some internal context to this which I'll send you in a DM

[dbkr]

So yeah, this is just how we line up overlay timelines: since it's just lining up events from two different rooms, there's not really much of an alternative. On that basis I think this is fine.