Fix Threads list stuttering

[bxdxnn]

Content

Optimize Threads list:

• Paginate using the diffs from SDK so that we won't call timelineItemContentFactory.create() on every row again when paginating
• Move item processing out of the main thread to fix animation stuttering
• Minor fix in ThreadsListView.kt to fix the effect restarting on every scroll frame
• Check the Threads availability in the view so that we can show the top bar icon instantly by just checking the feature flag, then show a hint text in the view if there are no threads available. Also improves UX/discoverability (when we roll-out this and the whole Threads feature) IMO, though maybe needs a product decision, but since this is behind a feature flag I guess it's OK.

Motivation and context

Fix the stuttering in the Threads list on opening/scrolling, along with other delays (huge especially on slow network/device).

Screenshots / GIFs

Tests

• Open Threads list and scroll it, should be much smoother now.

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• I am aware of the etiquette.
• This PR was made with the help of AI:
  • Yes. In this case, please request a review by Copilot.
  • No.
• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly defines what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[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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[bmarty]

Thanks for the PR!

I have tested the feature and it's working well.

For the new empty view, I let @mxandreas / @americanrefugee confirm that this is what we want, or provide an alternative design / wording if necessary. The threads icon will be always displayed and in case there are no threads in the room, this view will be displayed:

Thanks!

[jmartinesp]

A couple of comments about the changes. Also, if we want to have an empty screen, it might make sense to use an AsyncData wrapper for the thread list, and check whether its dataOrNull() value contains something instead of creating a new variable and exposing it in the state. There are several examples of this in the app.

[bxdxnn]

I've reverted the delay optimization as it's too complicated to do it properly ATM, something like matrix-org/matrix-spec-proposals#4360 is needed for it to work fast and correct but it seems to be WIP (?) and doesn't have an implementation yet.

[bxdxnn]

@bmarty FYI product/deisgn decision isn't required anymore as I've reverted the new screen/UX thing.

[bxdxnn]

Test failures doesn't seems to be related...