Components: Generate SplitButton

[JasonVMo]

Summary

Generate SplitButton with its canonical contract, implementation, tests, stories, platform validation, and explicit package export so default and related actions can share one control. The local spec is missing, and the secondary action region remains downstream of Menu and Popover.

Observed. split-button is entry split-button in the canonical catalog and has no
implementation in packages/agentic/components. Upstream describes it as: Action control pairing a primary action region with an adjacent chevron region. Covers Rounded and Circular shapes; the primary region fires the default action directly while the chevron region opens a Menu of related actions.

Goal

Generate the split-button component in
packages/agentic/components/src/components against the
canonical Fluent Flex spec, with the colocated coverage, stories, and platform
validation the package requires, and export it explicitly from
src/index.ts.

Stage

Stage 3 (production). Wave 3 - focus-orchestrated and composed.

Why it matters

Observed. The canonical catalog has 50 components and the agentic library has
generated 18. split-button is one of the 32 in the gap, enumerated in
the missing-components index.

Inferred. It is a composed or focus-orchestrated component whose blockers resolve once the wave 2 overlay stack and the focus decision land, so it is a direct measure of whether those foundations were built correctly.

Observed current state

Canonical spec and source

Field	Value
Canonical spec	plugins/components/skills/split-button/SKILL.md with usage.md, web/tokens.yaml, web/accessibility.md, and web/interaction.md, at x3-design/fluent-design @ d334acf
Upstream tier	atom
Upstream catalog key	split-button in component-map.yaml
Local staged spec	None. No specs/split-button folder exists; the spec must be ingested first
Local implementation	None. No src/components/split-button directory exists

Closest existing FURN or V1 implementation

• packages/components/Button/src/CompoundButton - The nearest two-region button in FURN, though it is a text-plus-secondary-text layout rather than a split action. Desktop token overrides are CompoundButtonColorTokens.win32.ts and CompoundButtonColorTokens.windows.ts; CompoundButton.ios.tsx, .android.tsx, and .mobile.tsx are mobile-only.
• packages/components/MenuButton - MenuButton.tsx for the chevron-opens-a-menu half; MenuButton.macos.tsx is a macOS-specific native path.

Note on platform canon. Where a legacy package has platform-specific files, the
desktop path (.win32, .windows, .macos, or the unsuffixed cross-platform file)
is the compatibility reference. .ios, .android, and .mobile variants are not
treated as the cross-platform canonical implementation.

Local dependencies

• src/components/button - Observed present (component).
• src/primitives/icon - Observed present (primitive).

Known blockers

Observed. There is no recorded blocker, because there is no local spec folder and therefore no
blockers.md. The absence of an ingested spec is itself the first blocker; see
spec-ingestion-and-generation.md.

Scope

• Ingest or promote the spec into the component directory in the generated shape
  (SPEC.md plus a spec/ folder), per
  spec-ingestion-and-generation.md.
• Establish the public contract first: variants, slots, native props, accessibility,
  interaction states, and platform behavior.
• Implement the required file structure from
  src/components/AGENTS.md:
  split-button.types.ts, split-button.styles.ts, use*.ts, use*Styles.ts, render*.tsx, and
  the assembly module with a displayName.
• Style from Flex tokens first and record genuine token gaps rather than substituting
  unrelated values.
• Export the component, its public types, and the state, style, and render stages
  explicitly from src/index.ts under component-qualified unstable names.
• Depends on menu for the chevron region's overlay.
• Model the two regions as separate accessible targets with independent handlers and independent disabled states.
• Decide whether the two regions share one focus stop or two.

Out of scope

• Changing the contract of an existing generated component, unless this task explicitly
  calls for it above; raise it as its own item.
• Building shared infrastructure owned by another task: overlay hosting
  (popover), roving focus (focus-zone.md), spec
  ingestion (spec-ingestion-and-generation.md),
  or the coverage contract (component-test-strategy.md).
• Migrating any legacy FURN consumer onto this component.
• Mobile platform support; the applicable upstream companion is web, retargeted to
  React Native for Windows and macOS.

Deliverables

• src/components/split-button/ implementing the full required file structure.
• SPEC.md and a spec/ companion folder with provenance recording the upstream path,
  SHA, and ingestion date.
• Colocated split-button.test.tsx, split-button.types.test.ts, and split-button.stories.tsx.
• Snapshot coverage where the visual contract is state-dependent, or a recorded
  exemption.
• Explicit exports from src/index.ts, with src/index.test.ts updated.
• A changeset, per the repository's version management rules.
• A recorded readiness level with its supporting evidence.

Required coverage, stories, and platform validation

Runtime and type coverage:

• Both shapes.
• The primary region fires its action and does not open the menu; the chevron region opens the menu and does not fire the action.
• Independent disabled states, if the contract allows them.
• Focus behavior across the two regions.
• Accessible names for both regions, and the expanded state on the chevron region.
• User style and handler forwarding, and that user styles are applied after component
  styles.
• Compile-time slot acceptance and any mutually exclusive props.

Stories: a typed Meta titled Components/SplitButton, an args-driven Default, a
grouped Overview when the contract has several axes, one focused story per variant
axis, and an interactive story for each stateful axis. Self-driving axes expose
default<State> as the control; externally driven selection is owned by the story with
React.useState. Identity-changing axes are not exposed as controls.

Platform validation: the component's stories render and behave correctly on macOS
and on React Native Windows Fabric through apps/storybook,
including hover, pressed, disabled, focus, optional-slot, and constrained-content
scenarios. Add a stable root testID if the component joins the automated smoke set.

Acceptance criteria

• src/components/split-button exists with the required file structure and a displayName.
• Every variant axis in the component's SPEC.md is implemented, or a deviation is recorded in the spec.
• The component styles from Flex tokens, with any genuine token gap recorded rather than approximated.
• Focus feedback renders through FocusVisual; no outline* props and no React Native Windows native focus visuals are introduced.
• State precedence is explicit: disabled wins over pressed, which wins over hovered.
• Every stateful axis is classified as self-driving or externally driven, and both paths are covered.
• Runtime, type, and story coverage satisfy the package coverage contract.
• The component, its public types, and its pipeline stages are exported explicitly from src/index.ts, and src/index.test.ts covers them.
• The component's stories render on macOS and on React Native Windows Fabric.
• yarn workspace @fluentui-react-native/components format, lint, build, and test pass, and the Storybook bundle succeeds for both desktop platforms.
• A readiness level is recorded with its supporting evidence.
• A changeset is included.

Dependencies and ordering

• Blocked by spec-ingestion-and-generation.md: the spec must be pulled from the pinned upstream revision before generation can start.
• Blocked by wave 2, either directly or through a wave 2 dependency listed above.
• Gated by component-test-strategy.md: the component must satisfy the coverage contract on arrival rather than being retrofitted.
• Gated by readiness-model.md: the target readiness level must be stated before the work starts.
• Consumes the Infrastructure workstream (storybook-e2e.md) for automated macOS and Windows Fabric validation; until it lands, platform validation is manual.

Risks and open decisions

1. Two adjacent pressables inside one visual control is a hit-testing and focus-ring hazard on Windows Fabric; the FocusVisual geometry must cover both regions correctly.
2. Whether upstream's Style axis applies independently to the two regions is not stated and needs a decision.

Evidence and references

• Upstream spec: plugins/components/skills/split-button/SKILL.md and
  components-index/SKILL.md
  at x3-design/fluent-design @ d334acf.
• Canonical implementations to follow: src/components/button
  for a styled higher-order component and src/primitives/icon
  for a primitive.
• Authoring rules: src/components/AGENTS.md and the
  agentic component authoring skill.
• Workstream context: Components README and
  the missing-components index.

Sources retrieved 2026-08-20.