Self-review of the lazy-load PR caught three Important findings:
1. IO observer was re-armed on every messages change. The previous
loadOlder useCallback depended on `messages`, so every live agent
push recreated it → re-ran the IO useEffect → tore down + re-armed
the observer. In a perf PR shipping to chat-heavy users, that's
the wrong direction. Fix: refs for the captured state
(oldestMessageRef, hasMoreRef), narrow loadOlder deps to
[workspaceId], and gate the IO effect on `messages.length > 0`
(boolean) instead of `messages` so it arms exactly once when data
first lands and stays armed across appends.
2. loadingOlder setState race. Two IO callbacks dispatched in the
same microtask (fast scroll, layout shift) could both pass the
`if (loadingOlder)` guard before React committed setLoadingOlder.
Fix: synchronous inflightRef set BEFORE any await, cleared in
finally; loadingOlder state stays for the UI label only.
3. Retry-button onClick duplicated the mount-effect body. Single
loadInitial() callback now serves both, eliminating the drift
hazard.
Coverage:
- 4 new tests bring the file to 8/8 (was 4):
- loadOlder fetches with limit=20 and before_ts=oldest.timestamp
- inflight guard rejects three concurrent IO triggers while a
deferred fetch is in flight (asserts call count stays at 2,
not 5)
- empty older response unmounts the sentinel (proxy for the
anchor-clearing branch in loadOlder)
- IO observer instance survives three subsequent prepends — same
object reference both before and after, no churn
- Both behavioural tests verified to FAIL on the prior code
(stashed ChatTab.tsx, ran them alone, confirmed both red), then
PASS on this commit. Pinning real regressions, not tautologies.
- IntersectionObserver fake captures instances + exposes
triggerIntersection() so the IO callback can be driven directly
from jsdom (no real layout / scrolling needed).
Test: vitest run src/components/tabs/__tests__/ → 39 passed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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