Two reds reproduce the reported jank:
- new agent, first prompt: the row paints at top=100 then jumps to 144 through
authoritative hydration, because the draft tab and the agent panel each render
their own AgentStreamView for the same logical row.
- long loaded timeline, submit: scrollHeight collapses 10793 -> 9111 mid-turn,
because splitOrderedTail reclassifies measured rows into the virtualized segment
where the virtualizer falls back to estimateStreamItemHeight.
Also restores centered timeline rows: the pagination anchor wrapper introduced in
#2490 was a plain block div, which broke alignSelf: center.
ceb06116f2