[Bug]: Parent-cancellation test can accept a late completion before closure

[ben-ranford]

Summary

The Linux race smoke job failed TestStaveTerminalParentCancellationPublishesIndeterminateOutcome because a late completion event returned nil instead of session.ErrSessionClosed. This differs from #1638's refresh-start timeout and #1637's resize-frame assertion.

Observed run: https://github.com/ben-ranford/lopper/actions/runs/34948806644/job/104314716672. PR1650 modifies suppression scanners and tests; it does not change Stave or this test.

Reproduction steps

1. Run the Linux race suite with the dev build-channel linker setting used by the CI smoke job.
2. In TestStaveTerminalParentCancellationPublishesIndeterminateOutcome, cancel the parent with a cause and finish the terminal run.
3. Verify the cancelled effect and shutdown events, then inspect the model's indeterminate-outcome state.
4. Send a late completed effect for the cancelled call and require ErrSessionClosed.

CI provides the observed failure. Three bounded targeted macOS race repetitions on the same PR head passed; no deterministic local schedule has been established.

Expected behavior

The shutdown contract defines when the session is closed. A late completion must not overwrite the cancelled/indeterminate outcome. The test must synchronize with that defined lifecycle state before making a closure assertion.

Actual behavior

The late send returned nil in CI after the preceding shutdown/state assertions passed.

Logs and output

--- FAIL: TestStaveTerminalParentCancellationPublishesIndeterminateOutcome (0.04s)
    stave_terminal_parent_cancel_test.go:86: late completion error = <nil>, want closed session
FAIL github.com/ben-ranford/lopper/internal/ui

Local targeted evidence: go test -race -ldflags '-X github.com/ben-ranford/lopper/internal/version.buildChannel=dev' ./internal/ui -run '^TestStaveTerminalParentCancellationPublishesIndeterminateOutcome$' -count=3 -timeout=90s -v passed all3 in1.678s.

Diagnosis and acceptance criteria

The cause is not proved. sendLopperEvent sends an event then waits for a sequence advance; test ordering, shutdown acknowledgement and eventual session closure may differ. Do not assume that every sequence change proves closure, and do not weaken the late-completion/outcome contract to make the test pass.

• Trace or control shutdown publication, reducer acknowledgement, session closure and late completion delivery to establish the failing ordering.
• Determine whether the defect is test synchronization, the bridge wait contract or production shutdown ordering; fix the demonstrated cause.
• Ensure late results cannot overwrite the indeterminate cancellation outcome.
• Preserve cancellation cause, backend cancellation, effect/shutdown ordering and error assertions.
• Add deterministic lifecycle synchronization and meaningful diagnostics; no arbitrary sleep, skip or discarded closure assertion.
• Repeat targeted normal/race tests on Linux and macOS, then run the repository CI gate.

Scope and scheduling

Targetv1.8.9, a focused cancellation lifecycle/test stability fix owned by the Stave work. No Stave implementation change is included in PR1650.

Version / commit

ed664c2.

Environment

GitHub Actions ubuntu-latest race smoke suite, Go1.27.1. Local macOS targeted checks use the same dev-channel linker setting. Other environments are not yet verified.