session-timeline-v1-regression-slice

Imported from OlyForge3D/PrintFarmer (.copilot/skills/session-timeline-v1-regression-slice/SKILL.md). Install upstream with npx skills add OlyForge3D/PrintFarmer --skill session-timeline-v1-regression-slice. Copyright stays with the author.

Context

Use this when PrintFarmer is stitching together a minimal print-session timeline from existing persisted sources instead of building a general audit/event platform.

Patterns

• Keep the backend gate centered on the composition seam:
  1. service test for merged event ordering and session association
  2. controller/integration test for the timeline endpoint contract
• Prefer the printer-scoped contract that actually shipped: GET /api/printers/{printerId}/session-timeline?take=N, returning recent sessions with nested mixed events rather than a flat synthetic stream the frontend must regroup.
• Include an orphan-incident boundary test: persisted FailureDetectionIncident rows with JobId = null should still attach when their timestamp falls inside the printer session window.
• Keep the frontend gate centered on operator-visible rendering:
  1. chronological mixed rows (state changes + failure incidents)
  2. auto-pause companion row when a failure incident paused the job
  3. loading/error/empty states for the timeline panel
• Reuse the failure-incident persistence triad as a dependency gate. If failure incidents stop being stored or lose job context, the session timeline becomes untrustworthy even if the timeline service still returns data.
• Treat backend/frontend contract mismatches as first-class regression seams. If backend composes sessions in one endpoint but frontend still assembles the view from older endpoints, QA should call that out explicitly.

Minimum Test Slice

Backend

• PrinterSessionTimelineServiceTests
  • event composition and stable ordering
  • orphan incident attachment by session window
  • newest-session-first behavior and take limiting
• PrinterSessionTimelineControllerTests
  • success path
  • 404 for missing printer or missing target resource

Frontend

• PrintSessionTimeline.test.tsx
  • mixed timeline row rendering in chronological order
  • incident confidence + snapshot affordance
  • loading and error states

Dependency Gate

• FailureDetectionIncidentHistoryServiceTests
• FailureDetectionControllerTests
• PrintFailureMonitorPersistenceTests

Anti-Patterns

• Testing only the new timeline endpoint without proving failure incidents still enter it correctly
• Treating printer-level recent history as equivalent to a single print session without a boundary test
• Writing only snapshot tests for the UI; prefer assertions on row order, labels, and links
• Ignoring frontend/backend contract drift because both sides "sort of" show timeline data