Scrolling up into unloaded content is janky

[cyrneko]

Describe the bug

When scrolling up, the viewport can "rubberband" or snap back a bit as previously unsynced content is loaded outside of the viewport. This makes scrolling more difficult.

Reproduction

1. Open a room
2. Scroll up the timeline until you hit unsynced/unloaded events
3. Keep scrolling (at a controlled speed, i.e with arrow keys, this is easier to see.)

Expected behavior

Scrolling should not rubberband the timeline's viewport

Platform and versions

- OS: Linux x86_64
- Browser: Mozilla/5.0 (X11; Linux x86_64; rv:150.0) Gecko/20100101 Firefox/150.0
- Sable: v1.16.0 (604661e)

Additional context

This is not replicable in upstream Cinny.