Agent-detail tabs break shell layout: mailbox hides the stream, tab bar clips

Context

Operator, live session 2026-08-17 ~18:40Z (screenshots on record): "is it intentional, that when i activate the mailbox tab, the live stream vanishes? 3rd tab header (Configuration) gets cut off." It is not intentional — and the truth layer is innocent, verified live before filing.

The Problem (mechanism verified via Neural Link, not guessed)

1. Activating the agent-detail Mailbox tab makes the fleet-level LIVE ACTIVITY stream vanish. Whitebox read at the moment of the symptom: ActivityStream reports mounted: true, hidden: false — the component lives and believes itself visible. The vanish is pure LAYOUT: the mailbox tab's content (a message list rendering up to a 50-item page) has unbounded height, inflating the detail panel / shared flex parent and pushing the activity region out of the viewport. The state layer tells the truth; the layout hides it — the inverse of every other bug this program hunts, and just as operator-visible.
2. The tab header row overflows the panel width: the third header renders as "Configurat" — no shrink, wrap, or scroll handling on the tab bar at the panel's width.

The Architectural Reality

• The detail panel (Status · Mailbox · Configuration tabs) and the cockpit's activity region share the shell's flex geometry; an unbounded child defeats the containment.
• The mailbox pane already paginates (1–50) — the DATA is bounded; only the RENDER height is not. Scroll belongs INSIDE the tab body, never on the shell.
• The density work (#15037) froze the activity stream's bounds precisely so the shell could rely on them — the detail panel needs the same discipline.

The Fix

1. Height containment on the detail panel's tab body: the mailbox (and any tab) scrolls internally; the panel's height is shell-governed; the activity region keeps its slot with EVERY tab active.
2. Tab bar width handling at panel width: shrink/ellipsize labels with full text on title/hover, or a scrollable header row — whichever the §04 bar's density ladder prefers; no clipped label.
3. Cheap regression guard: an e2e asserting the activity stream's client rect is non-empty while each detail tab is active, and all tab headers fit or are reachable.

Acceptance Criteria

• With Mailbox (and each other tab) active, the live activity stream remains visible in the shell (client-rect witnessed, not state-inferred — today state and pixels disagreed).
• Mailbox list scrolls inside its tab body; shell scroll unaffected.
• All three tab headers readable/reachable at the panel's shipped width; no clipped label.
• E2e regression per Fix 3.

Out of Scope

Mailbox row selectability + message detail (#17311/#17268 family) · pane information design (#17268) · the pop-out widget variant (verify it inherits the fix; if its geometry differs, file the delta).

Related

Epic #14560 (parent) · #15037 (density bounds precedent) · #17268 (design-pass thread, operator feedback ledger) · #17271 (origin session class)

Live latest-open sweep: REST created-descending, latest 8 checked 2026-08-17T18:45Z, no equivalent; A2A window clean.

Origin Session ID: 7ee47ccf-d1c7-469d-a75e-15cebf3b5ea5

Retrieval Hint: query_raw_memories("mailbox tab activity stream vanish layout mounted visible overflow tab header clipped")