CngxStepperHost

Root-level group whose subtree contains the active step, or null when the active step is a root-level step (e.g. a trailing Finish). The single focus-driven-collapse source: atoms read it via this contract, never the concrete presenter. Drives CngxStepGroup.isCollapsed, the strip aria-expanded binding, and visibleStripNodes.

true while a commit is in flight (commitState.status() === 'pending'). Drives the busy-disable on nav affordances. Keyed on the strict 'pending' status, consistent with the per-step busy gate.

true when selectNext would move - i.e. an enabled, linear-unblocked step follows the active one. Derived from the same predicates select() enforces, so a Next affordance can never drift from the navigation it gates.

true when selectPrevious would move - i.e. an enabled step precedes the active one. The single gate a Back affordance reads; never re-derive it from stepsOnly at the call site.

Reactive current/previous pair for the commit-state status. The organism's <span cngxLiveRegion> reads this tracker on every pending → success / error transition. Allocated once per presenter - consumers MUST read this rather than calling createTransitionTracker locally so the linkedSignal is shared.

true when the active step is the first one (no earlier step).

true when the active step is the last one (no later step).

Index of the most recently refused commit target, or undefined when no rejection is pending. Set by the rejection arm of select(); cleared by a successful re-pick of the same target or via clearLastFailed. Drives strip rejection decoration and the commitRolledBackTo chain in liveAnnouncement.

Label of the step selectNext would land on, or undefined when none follows. Resolved through the enabled-step traversal so a "Continue to {{ label }}" affordance announces the real target.

Origin captured at commit-window open - the safe-harbour the user is returned to on optimistic rollback. Resolves the origin label for the commitRolledBackTo(originLabel) phrase. Cleared on success; retained on rejection through the persistence window.

Label of the step selectPrevious would land on, or undefined when none precedes.

Step-only count - the length of stepsOnly.

Step-only flat projection (group nodes filtered out). Structural-equal via flatStepsEqual; the single source for any step-count / step-position lookup.

Contract: variant and skin organisms MUST read this signal directly. Local re-derivation (e.g. flatSteps().filter(...) inside a variant's computed) is rejected at review time - the projection's equality contract is the load-bearing invariant, and a local re-filter breaks the shape-stable short-circuit that downstream consumers rely on. Add lookup helpers as methods on this contract rather than re-deriving.

Flat strip projection under the focus-driven group-collapse policy: identical to flatSteps when groupCollapse is 'off', and with every non-active root-level group's children dropped (header node alone) under 'expand-active'. The strip @for reads this; stepsOnly stays the single counting / announcement source.

Contract: variant and skin organisms drive the strip off this projection, never re-derive it. Structural-equal via flatStepsEqual (each retained node keeps its flatSteps flatIndex), so a shape-stable re-emit short-circuits the strip render.