The timeline-virtualization acceptance gate is a same-harness delta
(header-arm longest-longtask <= B + 15ms), but the instrument that
produced the baseline was ad-hoc and never committed, so it evaporated
between sessions. Commit it so the gate's own instrument survives.

Measures main-thread longtasks during the first (cold) switch into the
600-message deep-history channel under 4x CPU throttle, windowed to the
300-row ceiling. Reports median-of-5 longest-longtask, run-to-run
spread, and total-longtask-time-in-window. Instrument, not a gate: the
only assertion confirms the switch exercised the mount under throttle.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

…ized timeline

A sticky header inside the scroll container drifts ~49px at scrollTop 0:
once older history prepends and the scroll restores, the header pins to
its clamped offset, but at the top it had been sitting at its larger
natural flow offset. The fix portals the header into a non-scrolling
overlay container outside the scroll element (mirroring the unread-pill
overlay), so it pins to a fixed offset regardless of scroll position and
cannot move as content prepends above the anchor. The per-scroll
re-render that resolves the topmost visible day stays localized to
VirtualizedList rather than forcing MessageTimeline to re-render on every
scroll.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

The timeline-virtualization port regressed scroll-anchoring: it dropped
the non-virtualized list's implicit "full content height exists at pin
time" invariant. Rows mount estimate-sized and measure to real height on
scroll, so the raw `scrollTo(scrollHeight)` mount pin landed short and
`scrollHeight` grew as off-screen rows measured.

Drive the mount bottom-pin through `virtualizer.scrollToIndex(lastIndex,
{align:"end"})` so TanStack lands the true last row through its own
measurement. Arm the existing settle-guard on smooth `scrollToBottom`
too — an animated jump is not atomic, so a mid-animation `onScroll`
latched a stale mid-history anchor that the ResizeObserver then restored.
Teach the prepend-restore loop to re-aim at the bottom when the user
abandons to bottom mid-restore, and the all-gone fallback to keep a
windowed-out anchor (vs. only pinning on genuine deletion).

The teleport spec's `scrollHeight <= baseline+100` setup proxy assumed
the non-virtualized contract (scrollHeight changes only on DOM adds); a
virtualizer grows `getTotalSize()` from lazy measurement alone. Replace
it with the direct in-flight signal the suite already keys on. Seed loops
that omitted `createdAt` collided on one whole-second stamp and sorted by
random UUID, so the asserted last/target row landed at a random index
often outside the virtualized window — make them monotonic to match the
file's own channel-intro seed precedent. No product-property assertion
text changed.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

…lization

The prior helper asserted symmetric balance (gapAbove == gapBelow within
1px), valid when intro, divider, and first row shared one flow layout. The
virtualization re-land moved the divider and first row into the translateY
track while the intro stays a flex sibling, so the two gaps are no longer
comparable quantities. The fix had collapsed the oracle to bare non-overlap
(>= 0 on both gaps), which gutted the layout-regression guard the test exists
to provide.

Source measurement showed the intro -> divider gap is the layout-controlled
8px and rock-stable, while the divider -> message gap is ~0 by construction
(virtualizer rows are back-to-back) plus MessageRow avatar/font render jitter,
genuinely variable run-to-run. So band the stable gap (8 +/- 2) as the real
guard and keep the variable one as a non-overlap reading-order check. Renamed
to match what it now verifies.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

The keyed scroll container remounts per channel, but the virtualizer and the
anchored-scroll ResizeObserver are owned by the parent and persisted across
switches, so on switch-back they kept pointing at the previous channel's
detached nodes. The mount-pin then fired scrollToIndex against a stale
virtualizer (scrollElement on a detached node), landing at the top instead of
the bottom, and the late-measurement bottom-chase never ran — so the
top-anchored channel intro painted when it should be windowed out.

Match the JS objects' lifetimes to the scroll node's: key TimelineMessageList
on channelId so the virtualizer remounts fresh, register it in a layout effect
so the parent's same-commit mount-pin reads the fresh instance, and add
channelId to the observer deps so it re-observes the live content node.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>