fix(desktop): restore fast middle-page scroll coverage setup (#155)

[oscarlehuu]

Problem

scroll-history.spec.ts — fast middle-page scroll settles with continuous mounted coverage — failed on main with:

expect(received).toBeGreaterThan(expected)
Expected: > 5608
Received: 3608

Issue #155 framed two opposite product causes (page-fetch shortfall vs virtualizer coverage). Discriminating evidence shows neither: the suite never reached the middle-scroll coverage loop.

Evidence (before fix)

Recorded at the setup prepend step on main:

Probe	Value
scrollHeight before live older emits	3608
mounted [data-message-id] rows	49 (initial ~50-event window)
older live emits	100
of those mounted immediately	0
scrollHeight 1s later	3608 (unchanged)
prepended settle text in DOM	none

Product path that rejects the harness setup:

// desktop/src/features/messages/lib/channelWindowStore.ts
// mergeLiveChannelWindowEvent — events below an open oldest boundary wait for paging
if (
  oldest &&
  (event.created_at < oldest.created_at ||
    (oldestPage.hasMore && compareRelayOrder(event, oldest) >= 0))
) {
  return current;
}

Unit contract already locks this: live rows below an open oldest boundary wait for paging.

Cause kind: HARNESS (not product page-fetch shortfall, not virtualizer coverage bug).

Approach

Buzz seam: channel window store live-merge + older-page pagination (mergeLiveChannelWindowEvent / pageOlderMessagesUntilRowFloor / top-edge sentinel).

• Seed older mock history with __BUZZ_E2E_PREPEND_MOCK_HISTORY__ before open.
• After open, wheel up so the real older-page fetch lands a genuine Virtua shift prepend.
• Keep the original middle-scroll continuous-coverage asserts (no weaken / skip / timeout raise).

Tests

Red → green (target):

# main
[smoke] fast middle-page scroll… FAIL  Expected: > 5608  Received: 3608
# this branch
[smoke] fast middle-page scroll… PASS  (3.1s)

Local collateral:

• scroll-history.spec.ts full file: 18 passed
• relay-reconnect.spec.ts: 13 passed, 1 failed at :617 (known baseline relay-reconnect.spec.ts:617 — pre-disconnect message disappears from the timeline after a 260-message reconnect backfill #154, sibling)
• channelWindowStore + timeline retention unit tests: 31 passed
• desktop typecheck + biome on touched spec: green

Docs

• docs/crew/STATE.md — note scroll-history.spec.ts:1281 — fast middle-page scroll settles with ~half the required mounted coverage #155 harness root cause and fix.

Risk

• Low: harness-only setup change. Product retention / virtualizer / live-merge behavior unchanged.
• If wheel-up fails to page on a slower runner, the existing scrollHeight poll still fails loudly rather than weakening coverage.

Exclusions

• Does not fix relay-reconnect.spec.ts:617 (relay-reconnect.spec.ts:617 — pre-disconnect message disappears from the timeline after a 260-message reconnect backfill #154).
• Does not change product eviction/retention bounds.

Fixes #155