* Add shared timeline overlay buttons and view contract
The TimelineOverlayButtons presentational component (unread bar, jump-to-bottom)
plus the shared TimelineView VM contract (types) and its i18n strings. Consumed
by the TimelineView virtualizer in a follow-up commit.
* Add shared TimelineView component (TanStack Virtual)
Headless TanStack-Virtual timeline driven from RoomTimelineViewModel: anchorTo:"end"
viewport stability across prepends/trims, cold-load cover, jump-to via scrollToOffset,
and visible-state reporting via the virtualizer's onChange. Adds the @tanstack/react-virtual
dependency and the virtual-core isValidAnchorItem patch (runtime + types).
* Use Compound spacing tokens in timeline overlay and clarify comments
- Use Compound spacing tokens for overlay margins/insets instead of raw px
- Drop the stale Virtuoso reference from the overlay CSS comment
- Note the real-world duration of COLD_CAP_FRAMES
- Describe each load phase in plain English
* Remove jargon and explain/expand things in simple terms.
* Use time rather than a frame count and improve description.
David LangleyandGitHub
badb35bd1f