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6. Time-travel in Story

Time-travel in Story rides Causa. Causa is mounted into the Story shell's right-hand pane by default (rf2-sgdd3) — its L1 ribbon (◀ ▶ ⏭) + L2 event list are the scrubber; the Trace tab is the cascade view; the Event-tab cascade view is the actions log. There is no Story-side scrubber UI any more; Story's contribution is the frame isolation that makes per-variant time-travel meaningful.

Why per-variant matters

Storybook's interaction recorder runs sequentially against one canvas. To compare two scenarios you re-mount the page.

Story's frame-per-variant isolation makes parallel time-travel natural. A workspace of four variants is four independent frames — each has its own app-db, its own dispatch queue, its own epoch buffer. Rewinding empty doesn't affect loaded or clicked-three-times or save-stubbed. You can rewind one cell while watching the play sequence in another.

When you select a variant in the sidebar, Story re-opens Causa scoped to that variant's frame — Causa's ribbon, event list, and detail tabs all bind to the selected frame's spine.

What you'd reach for it for

  • "My play sequence asserts something that's wrong at step 4. Why?" — open the variant in Canvas mode, run the play step-by-step, scrub backwards in Causa from the failing assertion. The epoch one back from the failure is the state the assertion ran against.
  • "I want to capture the canvas at three points in the variant's lifecycle." — :play it; the runtime records an epoch per step; navigate Causa's L2 event list to each, screenshot.
  • "I'm tuning a play sequence." — write the events, walk them in Causa, see what each one did to app-db. Edit. Re-run. The buffer is fresh each session.

The six failure modes, again

restore-epoch against a variant frame has the same six failure modes as against the host app's frame (see Causa chapter 3):

  • Unknown frame
  • Unknown epoch
  • Schema mismatch
  • Missing handler
  • Version mismatch
  • Concurrent-drain rejection

Causa renders the failure mode inline — same red toast, same :reason / :explain / :missing-id / :expected-:got payload. The runtime's contract is stable; Causa renders against it the same whether the frame is the host app's or a Story variant's.

When time-travel isn't the right tool

Two cases:

  • "I want to replay the variant from the start with a tweaked arg." That's not time-travel; that's re-run with override. Edit the arg in the controls panel; the variant re-runs from the start with the new arg. Time-travel is for examining what already happened, not for re-running with changes.
  • "I want to time-travel the trace bus, not the app-db." The trace bus is a log, not a state. To re-render the trace from a different angle, use Causa's Trace tab filters (the same :op-type / tag / free-text filters Causa offers across all hosts).

The point of time-travel here, as in Causa, is that the runtime already recorded the state. Story is just the lens that lets you compare two variants' epoch buffers side-by-side, each in its own frame.

Next: multi-substrate side-by-side.