chore: fix Checkbox indeterminate state

[francinelucca]

This pull request addresses a bug in the Checkbox component's handling of the indeterminate state. Previously, clicking the checkbox could cause the indeterminate state to be lost if the state did not change. The update ensures that the indeterminate state is consistently maintained after user interaction.

Before:

Screen.Recording.2026-04-27.at.11.42.17.AM.mov

After:

Screen.Recording.2026-04-27.at.12.21.04.PM.mov

Changelog

Changed

• Updated the handleOnChange event handler in Checkbox.tsx to explicitly reset the indeterminate property and the 'aria-checked' attribute to 'mixed' after a change event, preventing the indeterminate state from being lost on click.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 9b64f34

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[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.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[Copilot]

Pull request overview

Updates the Checkbox component so the indeterminate state is re-applied after every render, improving reliability of the underlying input’s indeterminate DOM property.

Changes:

• Removed the dependency array from the useIsomorphicLayoutEffect that sets checkboxRef.current.indeterminate.

Show a summary per file

File	Description
packages/react/src/Checkbox/Checkbox.tsx	Ensures indeterminate is applied on every render by running the layout effect without a dependency array.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2

[primer-integration]

⚠️ Hi from github/github-ui! The integration workflow could not find a canary version for the latest commit on this PR.

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