This repository contains the source code and documentation for Base UI: a headless, unstyled React component library.

• Source code for components and private utils is in packages/react/.
• Source code for public shared utils is in packages/utils/.
• Experiments are located at docs/src/app/(private)/experiments/. Use for creating demos that require manual testing in the browser.
• Public documentation is located at docs/src/app/(docs)/react/. Alter the docs where necessary when changes must be visible to library users.
• When creating public demos on the docs, refer to the hero demo for the given component and largely follow its styles (both CSS Modules and Tailwind CSS versions). Other demos may also contain relevant styling. Do not add custom styling beyond the critical layout styles necessary for new demos.

• The shared /base-ui-review skill lives in .agents/skills/base-ui-review/SKILL.md. Update that file when the Base UI review workflow changes.
• Claude Code discovers the same shared skill through .claude/skills/base-ui-review/SKILL.md, which delegates to the .agents copy.
• The review skill is opt-in: only run it when the user explicitly asks for it by name (/base-ui-review). Do not trigger it from a generic review request or after finishing a change.

• Always use the useTimeout utility from @base-ui/utils/useTimeout instead of window.setTimeout, and useAnimationFrame from @base-ui/utils/useAnimationFrame instead of requestAnimationFrame. Search for other example usage in the codebase if unsure how to use them.
• Use the useStableCallback utility from @base-ui/utils/useStableCallback instead of React.useCallback if the function is called within an effect or event handler. The utility cannot be used to memoize functions that are called directly in the body of a component (during render), so continue with React.useCallback in those scenarios.
• Always use the useIsoLayoutEffect utility from @base-ui/utils/useIsoLayoutEffect instead of React.useLayoutEffect.
• Always use the shadow DOM-safe utilities for DOM traversal and event targeting: closest, contains, getTarget, and activeElement. Always use the owner utilities ownerDocument and ownerWindow instead of global document/window lookups when the code is tied to a DOM node, including realm-sensitive checks such as instanceof.
• Avoid duplicating logic where necessary. If two components can share logic (such as event handlers), define the logic/handlers in the parent and share it through a context to the child; use the existing context if it exists.

• In CSS Modules demos (docs/src/app/(docs)/**/demos/**/*.module.css), use raw color values from the Tailwind @theme block in docs/src/css/index.css. For example, use oklch(14.5% 0 0deg) instead of var(--color-neutral-950).
• When using user-select: none; in CSS, also add -webkit-user-select: none; to support Safari. Tailwind's select-none class already includes this.

• Do not randomly cast (for example as any) if there are no type errors without doing so. Run pnpm typescript to verify types.
• Ensure your changes pass linting - run pnpm eslint.
• Ensure your styles pass stylelint - run pnpm stylelint.
• Ensure your changes are formatted correctly - run pnpm prettier.
• When you change a public component API (props or JSDoc), run pnpm docs:api.

• If a repository command fails because dependencies are unavailable, run pnpm i first and then retry the command.
• Run tests in jsdom env with pnpm test:jsdom {name} --no-watch such as pnpm test:jsdom NumberField --no-watch or pnpm test:jsdom parse --no-watch.
• Run tests in Chromium env with pnpm test:chromium {name} --no-watch such as pnpm test:chromium NumberField --no-watch or pnpm test:chromium parse --no-watch.
• Do not call await flushMicrotasks() directly after await render(...) when there are no interactions or state changes between them; render is already awaited, so that immediate flush is unnecessary.
• Do not group multiple expect() assertions in a single waitFor() callback. Use one assertion per waitFor() so retries are scoped to the specific condition that may change asynchronously.
• For locale-formatted text, use expect(element.textContent).toBe(expected) with the expected value formatted using the same locale and options as the component. When the component uses the runtime default locale, derive expectations using that default too (for example, new Intl.NumberFormat(undefined, options)); do not hardcode en-US or US-formatted strings in expected values. Pin a locale only when testing locale-specific behavior, and use it consistently for both the component and expectations. toHaveTextContent normalizes nonbreaking spaces in the rendered text but not in the expected string, causing failures in locales such as pt-BR.
• Use firePointer from #test-utils instead of fireEvent.pointer* whenever a test depends on event timing. fireEvent silently drops timeStamp, so the event inherits the environment's clock — the real one in a browser — and gesture velocity then depends on how long the runner took between calls. A lint rule enforces this.
• If you made changes to the source code, ensure you verify your changes by running tests (see above), and writing new tests where applicable. If tests require the browser because, for example, they require layout measurements, restrict it to the Chromium env by using it.skipIf(isJSDOM) or describe.skipIf(isJSDOM) (search other tests for example usage if unsure).
• Follow the established conventions in existing tests. Each file/component is tested with the filename name.test.tsx. For example, PopoverRoot.test.tsx is next to its source file PopoverRoot.tsx.
• Tests use Vitest APIs only: expect(), vi.fn(), and @testing-library/jest-dom DOM matchers. Do not use Chai- or Sinon-style matcher chains or spies.

• Commit messages follow the format [scope] Imperative summary (for example [popover] Fix focus trap). Choose scopes that mirror package or component names that were changed.
• Use [all components] scope for changes that broadly affect most components.

These guidelines apply only to errors thrown by public packages.

Every error message must:

1. Say what happened - Describe the problem clearly
2. Say why it's a problem - Explain the consequence
3. Point toward how to solve it - Give actionable guidance

Format:

• Prefix with Base UI:
• Use string concatenation for readability
• Include a documentation link when applicable (https://base-ui.com/...)

You MUST run pnpm extract-error-codes to update docs/src/error-codes.json every time you add or update an error message in an Error constructor.

Important: If the update created a new error code, but the new and original message have the same number of arguments and semantics haven't changed, update the original error in error-codes.json instead of creating a new code.

Before any changes, review CONTRIBUTING.md for more detailed guidelines on contributing to this repository.