Timeline: Replace ScrollPanel with Virtuoso POC

[langleyd]

What's in this PR?

A POC to learn more about the feasibility of using react-virtuoso for the timeline(whose use is looking promising for the simpler scrolling use cases of member list and room list).

This PR supports back pagination and crudely hide images for smooth scrolling, without the noticeable scroll jumps of our implementation when you get to the top or bottom of each page.

If you scroll fast enough you do see some white space before it renders, but we should be able to reduce this with further performance improvements to the individual components. We can also play with extending the viewport or use overscan of virtuoso to try reduce that.

If we use placeholders rather than hiding the images it would looks much less janky than in the video and you wouldn't get the existing scroll jump when scrolling stops the images pop in.

The implementation would be easier to write/read/test and probable more performant with the Message Panel Moved to FC(and MVVM) as well as components further down the tree.

virtuoso supports:

• aligning the items to the bottom when the viewport is not full
• sticky bottom for smoothly scrolling new messages in to view when you are near the bottom of the timeline.
• prepending data while maintaining scroll position(and appending).
• Good apis for keyboard navigation(as we have already applied in the memberlist use case)
• Good scrollTo apis for jumping to specific messages, top, bottom
• Good scrolling performance(as seen in the memberlist use case)
• Decent documentation with and lots of examples
• Tests written in playwright, which might bode well for us writing timeline tests

What does it look like?

1. Scrolling up with back pagination. You still need to wait for the data to load, with better caching that could be as smooth as going down though.
2. images disappear during scrolling
3. Scrolling down after the data is loaded is smoother and you can quickly get to any point in the timeline.
4. Images start to load in after a timeout

Screen.Recording.2025-05-14.at.10.25.05.mov

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)