Collapse long turns + auto-follow + jump-to-error on RunDetail (BEN-48)

[benSepanski]

Context

BEN-48: src/web/RunDetail.tsx rendered every turn in a <pre> with no max height. A run with 30+ turns or a turn carrying a large tool output became a several-thousand-pixel scroll, and live runs appended new turns below the fold with no way to follow.

TL;DR

Collapse tool turns to one-line previews and assistant turns above 12 lines to a "Show full turn" toggle, add an Auto-follow pill that pins the viewport to the newest turn on live runs, and add prev/next jump buttons over error events.

Summary

• New src/web/runDetailUtils.ts — pure helpers (turnLineCount, turnLineThreshold, shouldCollapseTurn, collapsedSummary, findErrorEvents, stepCursor, eventDomId). 20 unit tests in runDetailUtils.test.ts.
• RunDetail.tsx::TurnCard — turn content collapses by default when over its role threshold (1 line for tool, 12 for assistant), shows a Show full turn (+N more lines) / Collapse turn (N lines) button.
• RunDetail.tsx::TurnsSection — adds an Auto-follow pill (visible only on running runs). When on, the section's tail sentinel is scrolled into view on each new turn; a manual upward scroll silently flips it off (programmatic-scroll grace window suppresses self-cancel).
• RunDetail.tsx::TurnsSection — when any error event exists, ↑ prev / ↓ next buttons cycle a cursor and scrollIntoView the matching <li id="event-…">. Error rows in the events list are now tinted rose.
• ToolCalls <details> defaults closed when the JSON exceeds the assistant threshold; the rendered prompt <details> is unchanged.

Demo

n/a — automated cron run; no browser MCP in this environment. Manual smoke-test path: pnpm dev WORKFLOW.md --mock, open a long run, verify (a) tool turns show "first line + N more lines" with a Show-full toggle, (b) on a live run the Auto-follow pill keeps new turns visible and disables itself when you scroll up, (c) prev/next error buttons appear when errors are present and scroll to each event row.

Alternatives

• DOM-measurement-based maxHeight with overflow gradient — rejected: requires a useLayoutEffect + ResizeObserver per turn and the line-count heuristic matches the ticket's Show full turn (X lines) wording directly.
• IntersectionObserver on the tail sentinel for auto-disable — rejected: programmatic smooth-scroll briefly hides the sentinel, which would falsely cancel auto-follow. The scroll-direction + grace-window approach avoids that race.
• Server-driven "next error" navigation via the API — rejected: events are already in memory in RunDetail, so client-side cursor is one useState.

Test Plan

• pnpm all — typecheck + fmt:check + lint + test + eval (140 unit + 5 eval, all green)
• pnpm build:web — bundle builds (181ms)
• New runDetailUtils.test.ts covers per-role thresholds, line counting, summary slicing, error-event filtering (case-insensitive), and cursor wrap-around.

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