fix(app): keep root tab active for subagents

[opencode-agent]

Summary

• keep the root session tab active while viewing direct and nested subagents
• resolve tab identity synchronously from the shared session cache after card navigation
• fall back to full parent-lineage resolution for cold or incomplete session data
• add a nested-subagent regression covering active state and duplicate-tab prevention

Version comparison

The reported build is 0.0.0-beta-17727 (2a7d0729d0). The next/latest successful beta is 0.0.0-beta-17728 (f929856cdf). The only commit between them updates Nix node_modules hashes; there are no changes under packages/app, packages/client, or packages/session-ui. The different reproduction result is therefore state/timing-dependent, not a behavior difference between these two beta builds.

Root cause

Titlebar tabs represent root sessions, but the routed page identifies the currently viewed leaf session.

packages/app/src/components/titlebar.tsx previously matched a routed session tab in two steps only:

1. exact routed session ID
2. the fetched session's immediate parentID

That has two failure modes:

• Direct child navigation updates shared session data and then changes the route, but the titlebar starts a separate async fetch. Until that fetch resolves, currentTab() is undefined and the root tab renders inactive.
• Nested subagents have leaf.parentID === intermediateChildID, while the stored tab is keyed by the root ID. The root tab never matches, and reconciliation can add the intermediate child as a second tab.

Call stack and data flow

Subagent tool card
  packages/session-ui/src/tools/tool-renderer.tsx
  input: tool metadata.sessionID
  output: child session href + navigateToSession(childID)

SessionUIProvider.navigateToSession
  packages/app/src/pages/directory-layout.tsx
  data.session.sync(childID) -> shared session store
  navigate(child href)       -> LayoutRoute { type: "session", sessionId: childID }

Titlebar
  packages/app/src/components/titlebar.tsx
  input: LayoutRoute + shared session store + API fallback
  transform: leaf ID -> complete parent lineage -> root ID
  output: root-keyed currentTab / addSessionTab

TitlebarTabStrip -> TitlebarTabNav
  packages/app/src/components/titlebar-tab-strip.tsx
  packages/app/src/components/titlebar-tab-nav.tsx
  input: currentTab object identity
  output: data-active="true" on the root tab

The complete-lineage primitive already exists in packages/app/src/utils/session-route.ts, and the loaded-data resolver exists at packages/client/src/solid/data.ts as data.session.root(sessionID).

Fix

packages/app/src/components/titlebar.tsx now:

• reads the routed leaf and root synchronously from shared data when available
• validates async resource results against the current route
• walks the complete lineage with rootSession() when cached ancestry is incomplete
• uses the resolved root ID for both route matching and missing-tab reconciliation

Validation

Passed:

• bun typecheck (packages/app)
• bun run typecheck:e2e (packages/app)
• bun test --conditions=solid --preload ./happydom.ts ./src/utils/session-route.test.ts (4 passed)
• git diff --check

The focused Playwright test was attempted locally, but this shared host could not initialize Rolldown's Rayon thread pool (ThreadPoolBuildError: Resource temporarily unavailable) before the page fixture started. CI should execute the added regression.

Requested by: @Hona (Hona via Slack)