[jbeda]

Summary

• Settle Subagent, Team, and Parallel conversation-card chrome from their terminal lifecycle projections instead of leaving completed cards on the running ellipsis while a sibling remains active.
• Keep the later outer tool.result authoritative, and retain the newest genuinely pending tool in the running footer.
• Cover concurrent success/error completion, late authoritative results, and typed scrollback reconciliation.

Regression details

The card-header state split predates typed scrollback: from the first inline Subagent UI, subagent.end drove the terminal body while only the outer tool.result changed the header from … to ✓/✗. Historically those events were adjacent because each read-batch goroutine finalized and emitted its own result before the batch-level wait, so the mismatch was rarely visible.

Commit 409ea74e8 (feat: integrate contextual guardrails into execution and approvals) changed successful read-only execution to produce private records, wait for every sibling with wg.Wait(), and only then finalize results in call order. A fast foreground Subagent still emits subagent.end from inside its tool execution, but its outer result can now remain withheld behind a slow sibling for an arbitrarily long interval. That timing regression exposed the pre-existing Mecatui state bug.

The typed-scrollback extraction in e7e51c90b preserved the state split; it did not introduce the regression. This PR uses existing Subagent/Team/Parallel terminal projections to settle presentation during the newly long interval while retaining the later canonical result as authoritative. Generic read-only tools have no equivalent early lifecycle event and remain tracked by #1976.

Development stage

• Plan / Interface — Bounded/Architectural behavioral and exact-interface contract; no implementation
• Implementation — based on an approved, merged Plan / Interface PR
• Combined — compact one-task Bounded/Architectural exception; no separate plan PR
• Spike / Routine / Cleanup — acceptance-plan spine exempt; Spike evidence does not ship as-is

Contract linkage

• Work classification: Routine
• Classification rationale: Corrects Mecatui presentation using terminal lifecycle facts already present in the event stream; no engine, protocol, authority, persistence, or public interface changes.
• Decision record: None — no durable design decision changes.
• Human waiver of spine: No
• Acceptance plan: N/A — Routine bug fix
• Human decisions resolved and recorded: N/A — no new material interface decision
• Plan / Interface PR: N/A
• Approved commit baseline: N/A
• Combined/exemption rationale: Routine presentation regression fix

Interface conformance

• No public interface change. Typed scrollback now distinguishes an early terminal projection from the later authoritative tool result for presentation purposes only.

Issue relationship

Relates to #1976

Type of change

• Behavioral/interface plan
• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation/process
• Other (describe):

Test plan

Baseline checks

• Acceptance-plan checker
• Linting (task lint)
• Fast offline test suite (task test) — all relevant suites passed; the aggregate remains red on the unrelated existing ownership assertion at environment/microvm/repository_attachment_test.go:510, reproduced independently
• Pre-review race suite (task test:race)
• Offline demo (go run ./cmd/mecademo)
• Markdown changed: docs generation/link checks (task docs)
• User docs/user-facing behavior changed: site build (task site:build)
• Guarded engine API affected: compatibility check (task api:check)
• Intentional engine API change: task api:update + engine/CHANGELOG.md
• Landed plan: strict acceptance trace (task ac-trace-strict)
• Final implementation review: /panel-review

Additional focused verification:

• go test ./cmd/mecatui/ui ./cmd/mecatui/ui/internal/scrollback -count=1
• task test:golden

Changes

File	Change
cmd/mecatui/ui/tool_block.go	Render terminal delegation/parallel glyphs before the outer result arrives
cmd/mecatui/ui/internal/scrollback/tool.go	Track terminal projections separately from authoritative results
cmd/mecatui/ui/update.go	Apply Parallel terminal state and preserve remaining active siblings
cmd/mecatui/ui/conversation.go	Find the newest pending tool while avoiding snapshot hydration for non-tool cards
cmd/mecatui/ui/concurrent_tool_completion_test.go	Exercise concurrent lifecycle and late-result permutations

User-facing change

A completed fast Subagent or Parallel card now immediately shows ✓ or ✗ while a slower concurrent sibling continues running. The footer continues naming the remaining active sibling.

Special notes for reviewers

This deliberately does not change engine result timing. Generic read-only results are still retained behind the concurrent batch barrier; #1976 tracks the Bounded follow-up and acceptance plan.