Chat scroll-up infinite-scroll history paging broken (regression) — should use ORM cursor + IntersectionObserver

[joelteply]

Bug

Used to work: scroll the chat-widget up → loads more historical messages from the room. As of PR #950 baseline, scroll-to-top no longer pages older messages in. The widget renders a fixed initial slice and stops.

Joel called the fix shape directly: cursor-based paging via the ORM + an IntersectionObserver on a sentinel element at the top of the message list. When the sentinel becomes visible (user has scrolled to top), kick off the next-page fetch with before=<oldest visible message id>.

Why it matters

• Joel can't review prior conversation arcs from the UI.
• Personas already have full history via RAG, but Joel can't see what they're seeing.
• "It worked before" — regression, not a missing feature.

Fix shape

1. ORM side: data/list for chat_messages with { filter: { roomId, timestamp: { $lt: cursor } }, orderBy: [{ field: 'timestamp', direction: 'desc' }], limit: 50 }. Confirm cursor pagination is implemented in DataList/ORM; if not, add it (no fallback to offset — cursor is correct for messages).
2. Widget side: IntersectionObserver on a sentinel <div> at the top of the message list. On intersection, dispatch a "load older" action that fetches the next 50 with the cursor = oldest currently-rendered timestamp. Prepend results, preserve scroll position.
3. Edge cases: dedup against in-memory cache, stop fetching when result < limit, debounce so rapid scroll doesn't fire 10 fetches.

Out of scope for this issue

• Search-jump-to-message (separate issue)
• Date-divider grouping in the rendered list

Linked

PR #950 follow-up — UX-class regression Joel called out during paired QA 2026-04-23.