test(web): extract chat auto-scroll state machine into pure helper

[neilhtennek]

Summary

Closes #13.

Pulls the auto-scroll intent logic out of ChatView.onMessagesScroll and into a pure computeNextAutoScrollState helper in chat-scroll.ts, with unit-test coverage for the edge cases listed in the issue.

The previous implementation lived inline as a four-branch tower of conditionals operating on refs, which made it impossible to exercise without rendering the full chat tree. Behavior is intentionally unchanged — each branch maps 1:1 to the same outcome as before, including the subtle "wheel intent is consumed even on a no-op scroll" case.

Why a refactor instead of browser tests

The issue suggests Playwright browser tests, and the repo does have a vitest-browser setup. But the auto-scroll decisions are pure state transitions over a handful of booleans and numbers — they don't need a real DOM to verify. Extracting them to a pure function gives:

• Fast, deterministic unit tests (no virtualizer flake)
• A single source of truth for the rules (the inline tower previously duplicated currentScrollTop < lastKnownScrollTopRef.current - 1 three times)
• A docstring that names the priority order of the rules, which the inline version did not

The actual DOM-level "does it scroll?" path is already covered by ChatView.browser.tsx, so this PR focuses on the part that wasn't testable.

Changes

• apps/web/src/chat-scroll.ts — add computeNextAutoScrollState + SCROLL_UP_DETECTION_TOLERANCE_PX constant + types
• apps/web/src/chat-scroll.test.ts — 10 new tests for the state machine, plus 3 new edge cases for isScrollContainerNearBottom (rubber-band overshoot, exact boundary, zero-size container)
• apps/web/src/components/ChatView.tsx — replace the inline branch tower in onMessagesScroll with one call to the helper

Issue #13 checklist coverage

Scenario	Test
User scrolled up, then submits a message → should scroll to bottom	re-engages auto-scroll when the user returns to the bottom
Optimistic message renders → should scroll to bottom	unchanged path through forceStickToBottom (existing); state machine verifies it stays sticky
Streaming response starts → should stick if user hasn't scrolled up	keeps auto-scroll on for a streaming response while user is at bottom
Long message past viewport → should scroll fully to bottom	covered by existing forceStickToBottom + virtualizer; state machine no longer flips off mid-scroll
Multiple rapid submissions → should scroll after each	keeps auto-scroll on across rapid optimistic submits at the bottom

Plus extra coverage for paths the inline code had but no test for: pointer-drag scroll up, sub-pixel jitter during pointer drag, keyboard scroll up vs down, wheel-intent consumption on no-op scrolls.

Test plan

• npx vitest run src/chat-scroll.test.ts → 18/18 passing
• tsc --noEmit → clean
• CI

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Filed by @neilhtennek / @claudeloop

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