docs(combobox): a11y migration analysis docs

[nikkimk]

Description

Adds accessibility migration analyses for the 2nd-gen combobox family — a combobox plus the shared listbox and the two option-child components it composes. The combobox and picker share one swc-listbox, and options/option-groups can be children of a listbox, combobox, or picker.

combobox/accessibility-migration-analysis.md

• Documented ARIA roles/states/properties for swc-combobox — single combobox role on the inner <input>, aria-expanded/aria-controls/aria-activedescendant/aria-autocomplete, keeping aria-selected (chosen value) distinct from the active descendant (keyboard-active row)
• Documented the composition: a shared swc-listbox rendered in the combobox shadow DOM with swc-option/swc-option-group children projected into it by slot; aria-controls (input→listbox) is same-root, only aria-activedescendant (input→light-DOM option) is cross-root via ariaActiveDescendantElement
• Flagged an explicit open verification item: that ariaActiveDescendantElement resolves to a slotted option — directly and when nested inside an swc-option-group — across NVDA/JAWS/VoiceOver and Chromium/WebKit/Firefox, with a documented fallback if any combo does not honor slot projection
• Added a unique-value and distinct-sibling-label rule with a dev warning (value decoupled from label); folded in consumer-reported 1st-gen limitations (key/text separation, popup width/reflow, placeholder) as design motivation
• Cataloged known 1st-gen issues with dispositions (SWC-1195, SWC-1152, SWC-1127, SWC-2359, SWC-592, SWC-1255); identified the APG editable-combobox-with-list-autocomplete and grouped-listbox patterns

listbox/accessibility-migration-analysis.md (new component swc-listbox)

• Documented role="listbox" owned on the host via ElementInternals, factored out so combobox and picker share one listbox and it can also be used standalone (APG listbox pattern)
• Specified the two modes: standalone (focusable, own keyboard model and name) and embedded (rendered in a combobox/picker shadow DOM, not focusable, parent-driven active-descendant); single vs. multi-select (aria-multiselectable, LiveSelectionController); slotted option projection without re-rendering

option/accessibility-migration-analysis.md (new component swc-option)

• Documented role="option" owned on the host via ElementInternals — enabling the cross-root active-descendant reference and preserving per-option lang (fixes SWC-2359)
• Distinguished from swc-menu-item (option vs. menuitem); value decoupled from label and unique per widget; aria-disabled over native disabled; distinct-sibling-label rule + dev warning; parent may be a listbox, combobox, or picker

option-group/accessibility-migration-analysis.md (new component swc-option-group)

• Documented role="group" owned on the host, group naming, and the APG grouped-listbox pattern; sibling-label distinctness scoped within a group; never selectable/focusable/active; no nested groups; flagged as the extra-nesting case for the active-descendant verification

Registered all four docs in 03_components/README.md and regenerated navigation.

Motivation and context

The 2nd-gen migration is an opportunity to address known accessibility gaps, align with the latest WAI-ARIA Authoring Practices, and ensure the components meet WCAG 2.2 AA compliance. Owning role="option"/role="group"/role="listbox" on dedicated components (instead of borrowing sp-menu/sp-menu-item) is what lets the combobox and picker share one listbox and reference real author elements across shadow roots — removing 1st-gen's duplicate-render hack and its defects.

Related issue(s)

• resolves SWC-2453

Screenshots (if appropriate)

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Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed at the Accessibility Practices for this feature, see: Aria Practices
• I have added automated tests to cover my changes.
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

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Reviewer's checklist

• Includes a Github Issue with appropriate flag or Jira ticket number without a link
• Includes thoughtfully written changeset if changes suggested include patch, minor, or major features
• Automated tests cover all use cases and follow best practices for writing
• Validated on all supported browsers
• All VRTs are approved before the author can update Golden Hash

Manual review test cases

• Review the Combobox accessibility migration analysis
• Review the Listbox accessibility migration analysis
• Review the Option accessibility migration analysis
• Review the Option group accessibility migration analysis
• ARIA roles, states, and properties covered (combobox, listbox, option, option-group)
• Shadow DOM and cross-root ARIA considerations covered, including the shared-listbox composition and the ariaActiveDescendantElement slotted-option verification item
• Accessibility tree documented
• Keyboard interaction fully specified (active-descendant vs. standalone listbox models)
• Unique-value / distinct-sibling-label rule and dev warning covered
• Testing requirements defined
• Known 1st-gen issues cataloged with dispositions

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