fix(panel): skip live-append re-render while the full-message overlay is open (#85)

[rz1989s]

Fixes the churn half of #85 (part 1). Part 2 (widget conversation tail) is dispositioned on the issue per its own note.

Problem

While the full-message overlay was open on a LIVE timeline, every appended event fired renderShell(), rebuilding the entire panel — including a fresh SelectList for the overlay body, which resets the user's scroll position and selection on every append. Watching a live run while reading one of its messages was unusable.

Fix (surgical, at the churn source)

openRunTimeline's live-append subscription now skips renderShell() while fullMessageEvent is set:

• Overlay content comes from the stable fullMessageEvent — a rebuild renders identical output while churning scroll/selection.
• Nothing beneath the overlay is visible; no visual state depends on the re-render.
• Data stays current: runTimeline + selectedEventIndex still update per append; closing the overlay (esc) triggers the next full render, which picks up every accumulated append.
• Guard deliberately lives in the subscription, NOT at the top of renderShell — a top-level guard would also swallow the render immediately after setting fullMessageEvent (the open action), so the overlay would never appear.
• Tail-follow when the overlay is closed is untouched (append still rebuilds the timeline).

Why a guard in renderShell was rejected

Considered and rejected: if (this.fullMessageEvent) return at the top of renderShell breaks the open path (tl.onSelect sets fullMessageEvent then calls renderShell() — the early-return would eat exactly that call). It would also silently suppress store-refresh re-renders mid-overlay, entangling unrelated call sites. The subscription is the only site with the churn.

Tests (behavioral — real RunLog, real headless FleetPanel, real key driving)

• overlay open + append → messageBodyList identity stable, children untouched, runTimeline still grows
• overlay closed + append → timeline rebuilt (tail-follow preserved, pinned so the fix can't over-suppress)
• esc after appends → overlay closed, full render includes the events appended underneath

RED was verified first: the churn test failed against main with exactly the identity assertion. 824/824 (was 821), pnpm typecheck clean — both standalone. (The first cut passed tsx tests but failed typecheck — Panelish missing a key + a tautological assert; both gates caught it, per the house rule.)

Part 2 disposition

Widget conversation tail: the issue itself notes it's "only worth doing when a consumer asks; the live overlay covers the need today" — proposing close-wontfix-until-consumer on the issue.