Document prop merging conventions

[Copilot]

Defines consistent precedence when components and consumers provide the same prop.

Changelog

New

• Guidance for merging event handlers, class names, styles, and attributes.
• Examples for consumer overrides and component-owned attributes.

<button
  {...rest}
  className={clsx(classes.Example, className)}
  style={{...defaultStyle, ...style}}
/>

Changed

• None.

Removed

• None.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; documentation-only change

Testing & Reviewing

• Review the documented precedence rules and examples for clarity and consistency.

[changeset-bot]

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Latest commit: 7967c1b

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[joshblack]

@copilot add the mergeProps.ts utility from: https://github.com/primer/react/pull/8168/changes#diff-b21bbbcb2674bcbc6f8e205e1c9223efea602c2abea46bb150c1d34b8ee06d07 and include tests for it. Also add an ADR for prop merging conventions for me with this decision. Highlight any scenarios that I should be aware of that aren't included already

[Copilot]

@copilot add the mergeProps.ts utility from: https://github.com/primer/react/pull/8168/changes#diff-b21bbbcb2674bcbc6f8e205e1c9223efea602c2abea46bb150c1d34b8ee06d07 and include tests for it. Also add ...

Implemented in ba4584c. Added mergeProps with tests, ADR-025, and updated the style guide to use the utility. The ADR highlights refs, shallow style merging, defaultPrevented/stopPropagation, synchronous one-way cancellation, thrown errors, on* callback detection, zero- and multi-argument callbacks, and explicit undefined precedence.

[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

[joshblack]

@copilot remove this line, don't refer to ADRs as the style guide here should be the reference for everything moving forward

[Copilot]

Removed the ADR reference in 3254e14 so the style guide stands on its own.

[joshblack]

@copilot this is currently proposed and not adopted

[Copilot]

Updated ADR-025 to Proposed ⚠️ with implementation Pending ⚠️ in 7967c1b.