Add unit coverage for Neo.util.Style.compareStyles

Context

GitHub's current beginner guidance explicitly points newcomers toward the good first issue label. Neo's only open ticket with that label, #15124, is already assigned, so there is no unassigned entry point for the next contributor.

This ticket creates another deliberately bounded, self-contained contribution outside the v13.2 release work.

[!IMPORTANT] Reserved for first-time Neo.mjs contributors. The Neo maintainer team will not claim, edit, or implement this ticket through 2026-07-22. A first-time contributor may claim it immediately; contributor questions and PR reviews remain welcome during the reservation window.

Please comment before starting. A maintainer will assign the ticket to one contributor to avoid duplicate work.

Problem

Neo.util.Style.compareStyles() is the pure utility that turns old and new style values into the compact style delta consumed by Neo's VDOM diffing path. It accepts object or string inputs, reports added and changed properties, represents removed properties with null, and returns null when no delta exists.

The method has no focused unit spec. This is not theoretical risk: the regression documented in #6859 broke style removal when the new style value became absent, affecting UI filtering and producing misleading VDOM symptoms. The runtime fix landed, but its edge remains unpinned.

Architectural reality

• Runtime source: src/util/Style.mjs
• String normalization source: src/core/Util.mjs#createStyleObject()
• Direct runtime consumer: the style branch in src/vdom/Helper.mjs#compareAttributes()
• Canonical test destination: test/playwright/unit/util/Style.spec.mjs
• Established sibling pattern: test/playwright/unit/util/Json.spec.mjs and String.spec.mjs
• Structural pre-flight: fast-path; the proposed spec exactly matches the existing test/playwright/unit/util/ sibling role and naming pattern.
• This is pure single-thread utility coverage. It does not require a browser DOM, component instance, or VDOM integration fixture.

Fix

Add one focused Playwright unit spec for the existing Neo.util.Style.compareStyles() contract. Pin the current documented and runtime behavior without changing src/util/Style.mjs.

Contract ledger

Acceptance criteria

• Add test/playwright/unit/util/Style.spec.mjs using the existing util-spec setup and import pattern.
• Import src/Neo.mjs, src/core/_export.mjs, and src/util/Style.mjs.
• Cover added, changed, removed, and unchanged properties for object inputs.
• Cover all missing-side branches: both absent, old absent, and new absent.
• Preserve the #6859 regression edge: an absent new style maps every old style key to null.
• Cover string/string and representative mixed object/string inputs.
• Assert kebab-case string properties are normalized to camelCase and numeric values remain numeric.
• Assert object inputs are not mutated.
• Assert the old-absent branch returns an equal but different object reference from the new-style input.
• Assert unchanged styles and two absent inputs return null.
• Keep the change test-only. If documented behavior and runtime behavior appear to disagree, stop and ask on this ticket before changing src/util/Style.mjs.
• Run the focused spec: npm run test-unit -- test/playwright/unit/util/Style.spec.mjs
• Run the complete unit suite: npm run test-unit

Out of scope

• Refactoring or changing Neo.util.Style
• Changing Neo.core.Util#createStyleObject()
• Browser CSS validation or computed-style assertions
• Adding VDOM integration coverage
• Removing unrelated imports or performing broad utility cleanup
• Adding coverage for Neo.util.Date
• Any v13.2 release, Agent OS, Fleet Manager, or docking-feature work

Avoided traps

• Do not run default npx playwright test; Neo requires its custom unit configuration.
• Do not use browser DOM or toHaveCSS() assertions for this pure delta utility.
• Do not broaden the ticket into CSS parsing or normalization changes.
• Do not change runtime behavior merely to make a test easier.

Prior art and duplicate sweep

• Template precedents: #15117, #15118, and #15124.
• Historical regression anchor: #6859.
• Live latest-20-open, exact GitHub, Knowledge Base, local issue/discussion, and recent all-state A2A claim sweeps were refreshed immediately before creation at 2026-07-15T15:08:33.095Z; no equivalent ticket or active lane was found.

Decision Record impact

None. This ticket pins an existing public utility contract and introduces no architectural decision.

Origin Session ID: 2c46ce52-9c69-46a7-a8c9-4db937d2a341