Add smooth animation for new messages

[bxdxnn]

Content

Refactor the timeline row logic a bit to allow a smooth timeline animation for new rows

Motivation and context

Fixes #6966

Screenshots / GIFs

Tests

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[github-actions]

Thank you for your contribution! Here are a few things to check in the PR to ensure it's reviewed as quickly as possible:

• If your pull request adds a feature or modifies the UI, this should have an equivalent pull request in the Element X iOS repo unless it only affects an Android-only behaviour or is behind a disabled feature flag, since we need parity in both clients to consider a feature done. It will also need to be approved by our product and design teams before being merged, so it's usually a good idea to discuss the changes in a Github issue first and then start working on them once the approach has been validated.
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[github-actions]

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[jmartinesp]

This is an improvement from what we have, at the cost of making the timeline logic a bit more complex and presumably less performant - extracting read receipts when the timeline items change, I don't think it remembers anything to prevent new computations?

Also, it still has the issue about expanding a group of events overlapping with the items above it. It's not terrible, but it does look weird:

timeline.mov

I imagine there can be other issues like this, where components suddenly changing sizes can cause overlaps, but I couldn't find any, so maybe it's the only case where this could happen.

So I'm a bit torn: it feels smoother than my implementation in #7686, but it's also more prone to having animation issues.

[bxdxnn]

@jmartinesp OK, I'll fix the issues you've mentioned. Are you generally OK with this PR though?

[jmartinesp]

I'm not sure if splitting the timeline items and the read receipts is a good idea, but it seems like it's the only thing that could make inserting an event seamless: I tested some animations a bit more without splitting the events and read receipts, and while they looked better than before, there's a point where the previous event has lost its sent state and the new one receives it where the timeline item sizes change and there's more movement than I'd like.

Also, with the code in this PR there's a weird issue with read receipts that happens randomly: the animations for them are smooth at first, but then at some point they start being overlapped by the item above them and they kind of 'slide down' from them. I guess this too is caused by the reversed LazyList layout 🫤 :

Record_2026-09-10-17-22-29_ce4edcc1f0a419f86d172129647bdbcb.mp4

[bxdxnn]

@jmartinesp I've fixed the regressions you've mentioned, seems to be working so far. I'll regenerate screenshots to see what's up.

[jmartinesp]

This looks really good, the TimelinePlacementAnimationCoordinator trick is quite clever. I tried achieving something similar without separating the read receipts, but I couldn't, there was always some 'spring' animation because of the read receipts disappearing resizing the actual row.

I'm still not a fan of separating the read receipts into their own items, and I'm a bit scared of the complexity of the mapper, but the result is really good. I'd be in favour of merging this, but I'd want to know what @bmarty thinks first.

[codecov]

Codecov Report

❌ Patch coverage is 81.29252% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.87%. Comparing base (8101abf) to head (ff1d9e6).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
...features/messages/impl/timeline/TimelineLazyRow.kt	73.33%	13 Missing and 11 partials ⚠️
...id/features/messages/impl/timeline/TimelineView.kt	89.47%	7 Missing and 5 partials ⚠️
...imeline/components/TimelineItemGroupedEventsRow.kt	60.00%	5 Missing and 1 partial ⚠️
.../timeline/TimelinePlacementAnimationCoordinator.kt	55.55%	3 Missing and 1 partial ⚠️
.../timeline/components/ActiveCallTimelineItemView.kt	80.00%	0 Missing and 2 partials ⚠️
.../timeline/components/TimelineItemCallNotifyView.kt	80.00%	0 Missing and 2 partials ⚠️
...ssages/impl/timeline/components/TimelineItemRow.kt	60.00%	2 Missing ⚠️
...s/impl/timeline/components/TimelineItemEventRow.kt	88.88%	0 Missing and 1 partial ⚠️
...l/timeline/components/TimelineItemStateEventRow.kt	88.88%	0 Missing and 1 partial ⚠️
.../components/receipt/TimelineItemReadReceiptView.kt	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7688      +/-   ##
===========================================
- Coverage    80.90%   80.87%   -0.04%     
===========================================
  Files         2780     2782       +2     
  Lines        81899    82065     +166     
  Branches     11284    11324      +40     
===========================================
+ Hits         66264    66369     +105     
- Misses       11290    11334      +44     
- Partials      4345     4362      +17     

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[jmartinesp]

We've discussed this with the team, and given we're about to start working on a multi-message selection feature that could conflict with this PR, we'll implement that first and then try to figure out if this solution still fits, since having the read receipts as a separate timeline item could be a problem in this case.