Task 01: Make Shared-Environment Session Handoff Atomic

Acceptance

Switching between tasks in the same environment adds the incoming task's session panel to the outgoing session panel's group and tab index before closing the outgoing panel.
Closing the outgoing task's final session panel cannot destroy the center group or change the Dockview root orientation.
The existing active-panel policy is preserved when Files, Changes, or another non-session panel is active.
Stale session panels are removed, the incoming session panel is unique, and existing cross-environment restoration behavior remains unchanged.
Reloading after the task switch restores the same healthy horizontal-root layout and right-column vertical split.

TDD Sequence

RED: add a session-tabs unit regression whose fake Dockview destroys a group when its final panel closes; prove the current close-before-add ordering loses the group.
RED: add a desktop Playwright regression that switches between two tasks sharing an environment and asserts the live and reloaded Dockview tree semantics.
GREEN: anchor the incoming session panel in the first stale session panel's live group and tab index before stale cleanup.
REFACTOR: remove any duplication that can be shared safely with cross-environment stale-session replacement, while preserving focused tests.

Verification

cd apps && pnpm --filter @kandev/web test -- components/task/dockview-session-tabs.test.ts
cd apps && pnpm --filter @kandev/web test -- components/task/dockview-session-tabs.test.ts lib/state/dockview-env-switch-action.test.ts
cd apps/web && pnpm run typecheck
cd apps/web && pnpm e2e:run tests/layout/saved-layout-session-isolation.spec.ts

Files likely touched

apps/web/components/task/dockview-session-tabs.ts
apps/web/components/task/dockview-session-tabs.test.ts
apps/web/e2e/tests/layout/saved-layout-session-isolation.spec.ts
apps/web/e2e/pages/session-page.ts if the tree assertion belongs in the shared page object

Dependencies

None.

Inputs

docs/specs/ui/requirements/task-layout-profiles.md, especially Persistence guarantees and the shared-environment task-switch scenario.
The atomic stale-session replacement in apps/web/lib/state/dockview-env-switch.ts.
The existing same-environment early return in apps/web/lib/state/dockview-store.ts.

Output contract

Report the exact failing regression, the panel event order before and after the repair, files changed, unit/typecheck/E2E command results, and any remaining ambiguity for layouts contaminated with stale sessions in multiple groups.

Result

RED proved that the prior handoff emitted only close:session:outgoing, removed group-center, and left no incoming session panel.
GREEN emits add:session:incoming before the outgoing close, preserves group-center, and retains the right-side Files or Changes above Terminal split.
Focused unit tests, TypeScript typecheck, and the production-build Chromium regression passed on 2026-07-27.

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