[amanabiy]

Description

Stacked on #5178 (merged).

Why: We're adding the OptionList component. It needs the same list behaviour as Select and Multiselect, but that code lives in Select's private folders. Instead of OptionList depending on Select's internals, the three components will share one internal list engine in src/internal/components/listbox/. We're getting there in small steps that are easy to review and revert:

1. refactor: Move list rendering and helpers into internal listbox #5178 Move the shared list helpers out of src/select (no behaviour change)
2. refactor: share selection logic between Select and Multiselect #5179 Add shared selection models and use them in Select and Multiselect ← this PR
3. feat: Add OptionList component #5180 Add OptionList on the new engine (Select and Multiselect unchanged)
4. Later: move Select, then Multiselect, onto the engine, and remove the old Select-only code

What: One place for the selection rules, instead of copies in each component.

• New: singleSelection and multiSelection in src/internal/components/listbox/selection/. Given the current selection, they return the next selection when the user picks an option, and the checkbox state of group headers and select-all.
• Multiselect: option toggle, group toggle and select-all now come from multiSelection (removes about 40 lines of its own toggle logic).
• Select and Multiselect: group header and select-all checkbox state in useSelect now comes from the model, replacing the copy of the same formula.

No behaviour change.

Related links, issue #, if available: n/a

How has this been tested?

• Existing Select, Multiselect and PropertyFilter unit and integration tests pass unchanged.
• New tests for the models, the order of options in onChange, and group and select-all checkbox state in Select and Multiselect. The new Multiselect order test passes on the old code too.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[Copilot]

🟢 Approval recommended

The isolated models have focused unit coverage, introduce no production callers, and have no identified blocking issues.

No open issues 🎉

What changed in this PR

Adds shared internal listbox selection models for the upcoming OptionList component and later Select/Multiselect migration. This builds on #5178 without changing existing component callers or runtime behavior.

Changes:

• Defines a shared selection-model interface.
• Adds single- and multi-selection logic, including group and select-all state.
• Adds focused unit tests for toggling, filtering, disabled options, and selection edge cases.

File	Description
src/​internal/​components/​listbox/​selection/​single-selection.ts	Implements single-selection behavior and group state.
src/​internal/​components/​listbox/​selection/​multi-selection.ts	Implements option, group, and select-all toggling.
src/​internal/​components/​listbox/​selection/​interfaces.ts	Defines the shared selection contract.
src/​internal/​components/​listbox/​selection/​__tests__/​single-selection.test.ts	Tests single-selection behavior and edge cases.
src/​internal/​components/​listbox/​selection/​__tests__/​multi-selection.test.ts	Tests multi-selection, filtering, and disabled-option handling.

_{Review effort: Balanced}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.73%. Comparing base (b3106e4) to head (228673d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5179   +/-   ##
=======================================
  Coverage   97.73%   97.73%           
=======================================
  Files         996      998    +2     
  Lines       32222    32236   +14     
  Branches    11909    11909           
=======================================
+ Hits        31493    31507   +14     
  Misses        684      684           
  Partials       45       45           

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[Copilot]

🟢 Approval recommended

Static review supports behavior-preserving extraction with regression coverage and no blocking issues; tests were not executed during this review.

No open issues 🎉

_{Review effort: Balanced}