Third step of the virtualization plan. Pure refactor — no UI change, no
virtualization yet. Prepares the timeline for row-level windowing.
- Introduces `TimelineRow`, a discriminated union of `session-separator`,
`lead-thought-group` (pinned and non-pinned), `compaction-divider`,
and `message-row`. Each row maps 1:1 to a single visual element.
- Adds a `renderRows` useMemo that walks `timelineItems` once and emits
atomic rows, hoisting session separators out of the Fragment bundle
that used to pair them with their owning item. This is the shape a
windowing layer needs: each row measurable and addressable
independently.
- Extracts a `renderTimelineRow(row)` helper that switches on `row.kind`
and returns the same JSX the previous inline render produced. Logic
per kind is identical — keys, memoization, collapse props, pinned
thought "live" semantics — so there is no visual diff.
- The render body collapses from two blocks (pinned + `.slice().map()`)
into a single `renderRows.map(renderTimelineRow)` call.
Follow-ups will virtualize `renderRows` with measured row heights and
tighten observer/animation wiring; pagination, collapse state, zebra
striping, and `groupTimelineItems` are untouched.
Second step of the virtualization plan. No virtualization yet. This PR
makes IntersectionObserver-based visibility tracking correct inside
scroll containers (sidebar, bottom-sheet), which is a prerequisite for
virtualizing the timeline.
- Introduces `TimelineViewport` — a grouped contract passed as a single
`viewport` prop on `ActivityTimeline`. Holds `scrollElementRef`,
`observerRoot`, `scrollMargin`, and `virtualizationEnabled`.
- `MessageRowWithObserver` and `LeadThoughtsGroupRow` now create their
`IntersectionObserver` with `root = observerRoot?.current ?? null`
instead of defaulting to the document viewport. Unread marking now
fires when rows enter their real scroll parent.
- `MessagesPanel` resolves the active scroll owner from `position`
(inline from parent ref, sidebar from `sidebarScrollRef`, bottom-sheet
from `bottomSheetScrollRef`) and passes it into ActivityTimeline.
- Tests: stubs `IntersectionObserver` to capture `options.root` and
asserts null when no viewport is passed, and the provided element when
`viewport.observerRoot` is set.
`scrollMargin` and `virtualizationEnabled` are included in the contract
but not consumed yet — they land in follow-up PRs (#4/#5).
Replace the per-item backward scan that located the most recent session
anchor with a single forward pass via useMemo.
Before: for every timeline item the render loop walked backward until
it found a lead-thought anchor, so N items produced up to N * N anchor
lookups on every render pass.
After: a single O(n) sweep builds previousSessionAnchorByIndex; render
time lookup is O(1). getItemSessionAnchorId is hoisted to module scope
so it is not recreated per render.
Behavior is unchanged. The three existing separator tests still pass,
and four new cases cover three-session transitions, long runs of
non-anchor items between thought groups, consecutive same-session
thoughts, and single-item lists.
