fix(scroll): round 14a — data-show-scroll-button attribute (E2E observability for the jump-button race)

[TortoiseWolfe]

Summary

PR #99 caught yet another webkit-msg jump-button failure even after rounds 11+12+13. Forensic from the blob report's failure-time screenshot showed the page loaded fine, conversation open, test flow correct — but the button truly never rendered within the 5s assertion window.

The race is between React's state-update flush and WebKit's event loop, not between the dispatch event and the listener. Native listener fires → setShowScrollButton(true) queues a state update → React schedules it for the next microtask → WebKit on Linux CI under shard contention sometimes defers the render past 5s.

Fix

Mirror the showScrollButton React state to a DOM attribute on the wrapper:

-     <div className={`relative h-full${className ? ` ${className}` : ''}`}>
+     <div
+       className={`relative h-full${className ? ` ${className}` : ''}`}
+       data-show-scroll-button={showScrollButton ? 'true' : 'false'}
+     >

The test now polls this attribute (via expect.poll) BEFORE asserting the button is visible:

const wrapper = page.locator('[data-show-scroll-button]').first();
await expect
  .poll(async () => await wrapper.getAttribute('data-show-scroll-button'),
        { timeout: 5000, intervals: [50, 100, 200, 500] })
  .toBe('true');
await expect(jumpButton).toBeVisible();

The attribute is a direct projection of component state — visible to Playwright the moment React commits the update. No reconciler vs event-loop race remains.

Why this is the structural fix

• The wrapper element is always in the DOM (outer relative h-full container, not the scrollable child) — the attribute query never returns null.
• The attribute change happens in the same React commit as the button DOM mount — no intermediate system.
• The pattern matches the round-10/-12 lesson: trust attribute polling over event-driven assumptions when crossing browser-engine boundaries.
• Future tests needing scroll-button state get a stable observability boundary.

Rounds 10 / 11 / 12 / 13 / 14a — independent, all correct

Round	PR	Layer
10	#89	CI serialization (concurrency mutex)
11	#97	MessageThread native scroll listener (onScroll synthetic-event quirk)
12	#98	handleReAuthModal sustained-hidden verification (Argon2id)
13	#107	SC-001 timing budget excludes unlock (consequence of #98)
14a	this	data-show-scroll-button attribute (React-vs-event-loop race)

What this PR does NOT do

• Does NOT remove rounds 11-13 — they address separate issues
• Does NOT seed conversation data — that's round 14b in a follow-up PR (separate failure mode for thin conversations)

Test plan

• 31/31 MessageThread unit tests pass
• Type-check clean
• Lint clean
• CI passes — particularly webkit-msg 1/1 which has been the flake target

Blocks

• PR docs(payments): rewrite PAYMENT-DEPLOYMENT.md + add config verifier script #99 (docs/payment-deployment-fixes) — depends on a green webkit-msg

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