[Round 14b] Seed deterministic conversation data for messaging-scroll spec (~2h, follow-up)

[TortoiseWolfe]

Background

Round 14a (PR #108) added data-show-scroll-button attribute observability to fix the React-render vs WebKit-event-loop race that rounds 11-13 had partially mitigated.

This issue tracks the deferred Round 14b work: seeding deterministic conversation data.

Why this is now low-priority (was originally planned alongside 14a)

After Round 14a landed, the WebKit timing-race symptom is gone. The remaining argument for 14b is about deterministic data shape, not flake elimination:

• The test (messaging-scroll.spec.ts:266 T007-T008) currently uses "whichever conversation is first by Most Recent." That conversation could have 5 messages or 500.
• For thin conversations, the test silently skips the assertion (if (scrollInfo.distanceFromBottom > 500) is false). It "passes" but verifies nothing.
• For thick conversations, the test asserts correctly.

The test isn't broken without 14b — it's just non-deterministic about whether the assertion actually runs. With 14a's observability fix, when the assertion DOES run, it's reliable.

What 14b would do

Add a test.beforeAll(...) hook in messaging-scroll.spec.ts that seeds 30 messages into the test conversation via the Supabase admin client. Pattern mirrors tests/e2e/auth.setup.ts. 30 messages × 80-120px per message = 2400-3600px of scroll height, guaranteeing the assertion always enters the if branch.

Math

• Test scrolls scrollHeight - clientHeight - 600 pixels up
• Then asserts button visible IF distanceFromBottom > 500
• For the assertion to run reliably, scrollHeight - clientHeight needs to exceed 600 + 500 = 1100px
• At 80-120px per message: ~14 messages minimum, 30 messages for comfort

Acceptance

• beforeAll hook in messaging-scroll.spec.ts seeds 30 messages into the test conversation
• Seeded messages have timestamps spread over several hours (not all at once — realistic data)
• Cleanup hook in afterAll (consistent with other messaging specs)
• T007-T008 reliably enters the assertion branch in all 3 browsers
• No regression in other messaging-scroll tests

Related

• Round 14a: PR fix(scroll): round 14a — data-show-scroll-button attribute (E2E observability for the jump-button race) #108 (merged) — the actual flake fix
• The shared auth.setup.ts pattern for admin-client seeding

🤖 Created as follow-up after round 14a validation in PR #108