[Dmytro Kirpa (dmytrokirpa)]

Previous Behavior

There is no headless DatePicker. The DatePicker base hook supplies date state and popup requests,
but the styled renderer cannot provide an unstyled Input/Calendar integration.

New Behavior

• Implement stable DatePicker, useDatePicker, renderDatePicker, and related props/slots/state
  and event types through @fluentui/react-headless-components-preview/date-picker.
• Reuse DatePicker Preview's base state for controlled/uncontrolled dates and visibility, editable
  text, required/boundary/restricted-date validation, formatting/parsing, and callback semantics.
• Compose headless Input and the pending headless Calendar family. Preserve the native input ref,
  Field associations, consumer cancellation/slot refs, keyboard navigation, and month/year switching.
• Reuse headless Popover's native modal-dialog lifecycle and CSS-anchor positioning for same-document
  top-layer rendering, inert outside content, dismissal, and focus restoration. No styled fallback,
  default icons, Griffel styles, Portal, or new Tabster behavior is introduced.
• Reserve Input-wrapper presence attributes data-open, data-disabled, and data-invalid.
  Native dialog state and DatePicker-owned selection/boundaries cannot be overridden by slot props.
• Add conformance/unit/browser coverage, five CSS Module stories (including initially open hydration),
  public export map/API report, focused/all-components bundle fixtures, and a minor Beachball entry.
• Narrow the shared data-attribute helper's boolean overload to '' | undefined, without changing
  its runtime behavior.

Dependencies and intentional exclusions

Blocked draft: this is now an implementation, not the former documentation-only placeholder.
It was developed and validated against pending headless Calendar
#36752 (4a32a0ca54dd2709e84ee2cc6343612e64ddfeac) and standalone Calendar
#36917 (ba6a0e9db7).

Those prerequisite sources, stories, exports, generated Calendar API report, and change files are
not committed in this PR. A clean checkout of this stack is not expected to build until the
prerequisites merge and the stack is rebased. Calendar remains outside the DatePicker GitHub stack.
This PR follows public-preview publication #36918; it does not duplicate that PR.

The native dialog intentionally excludes mountNode and inlinePopup. Native dialog/CSS-anchor
support and native focusgroup support (or an application-supplied focusgroup polyfill) are required.
For editable inputs, openOnClick={false} permits typing without opening a modal; the trigger and
Enter/ArrowDown remain available. Callers own styling and visual accessibility.

Validation

All runtime/build evidence below was collected with the exact temporary prerequisites present.
They were removed before committing this PR.

• NX_DAEMON=false yarn nx run react-headless-components-preview:test --runInBand --runTestsByPath src/components/DatePicker/DatePicker.test.tsx --skipNxCache — passed: 26 tests, including conformance/axe, selection, validation, canceled keyboard/pointer events, controlled requests, disabled state, refs, Field, and server rendering.
• NX_DAEMON=false yarn nx run react-headless-components-preview:e2e --excludeTaskDependencies --browser chrome --config retries=0 --spec src/components/DatePicker/DatePicker.cy.tsx --skipNxCache — passed: 10 Chrome tests, including focus return/no reopening, native modal inertness, backdrop/Escape, rejected controlled closes, iframe ownership, view switching, month-only selection, editable validation, and RTL.
• NX_DAEMON=false yarn nx run-many -t type-check -p react-headless-components-preview react-headless-components-preview-stories --excludeTaskDependencies --skipNxCache — passed.
• NX_DAEMON=false yarn nx run react-headless-components-preview:lint --command='yarn run -T eslint src/components/DatePicker src/date-picker.ts src/utils/toDataAttributeValue.ts' --skipNxCache — passed.
• NX_DAEMON=false yarn nx run react-headless-components-preview-stories:lint --command='yarn run -T eslint src/DatePicker' --skipNxCache — passed.
• NX_DAEMON=false yarn nx run react-headless-components-preview:build --excludeTaskDependencies --skipNxCache — passed: ESM/CommonJS, declarations, API extraction, and CJS declaration generation.
• NX_DAEMON=false yarn nx run react-headless-components-preview-stories:build-storybook --excludeTaskDependencies --skipNxCache — passed; existing Storybook asset-size warnings remain.
• NX_DAEMON=false yarn nx run react-headless-components-preview-stories:test-ssr --command='yarn run -T test-ssr ./src/DatePicker/**/*.stories.tsx' --skipNxCache — passed: all five stories, including an initially open native dialog, SSR and Chrome hydration.
• NX_DAEMON=false yarn nx run react-headless-components-preview:bundle-size --excludeTaskDependencies --command='yarn run -T monosize measure --fixtures DatePicker.fixture.js --debug' --skipNxCache — passed: 71.616 kB minified, 22.5 kB gzip. Debug bundle inspection found no Griffel/icons/Tabster runtime or styled Calendar/Input implementation.
• yarn workspace @fluentui/react-headless-components-preview pack --out /Users/dmytrokirpa/.copilot/session-state/c4c94c91-a3cc-4e00-b996-2ff82c856e61/files/react-headless-components-preview.tgz — passed; packed export targets, rewritten workspace ranges, and ESM/CommonJS APIs passed the session smoke script.
• yarn beachball change --package @fluentui/react-headless-components-preview --scope packages/react-components/react-headless-components-preview/library --since HEAD --no-fetch --no-commit --type minor --message 'Add an unstyled DatePicker with shared date validation, headless Calendar composition, and native modal-dialog behavior.' — passed; package-scoped change file included.
• Shared Calendar regression validation — 706 Calendar unit tests and 14 styled DatePicker Chrome tests passed; the keyboard callback-order fix is committed only in fix(react-calendar-preview): preserve popup focus and compat geometry #36917.
• Built Storybook interaction checks — default/controlled selection and close, controlled status updates, editable validation, disabled state, and light/dark surface theming passed.

Manual VoiceOver/NVDA/JAWS checks, full browser/zoom acceptance, and CI visual approval remain
pending. Headless React 17/18 integration runs have not been performed in this change. Nothing
has been merged or published to npm.

Related Issue(s)

• Final layer of the DatePicker stack beginning at feat(react-datepicker-preview): scaffold package and DatePicker component #36914.
• Follows feat(react-datepicker-preview): prepare public preview publication #36918.
• Blocked by feat(react-headless-components-preview): add headless Calendar #36752 and fix(react-calendar-preview): preserve popup focus and compat geometry #36917; no separate linked issue.