perf(ThemeProvider): Reduce unnecessary renders and effect cascades

[mattcosta7]

Summary

Performance improvements to ThemeProvider in both @primer/react and @primer/styled-react by replacing useState + useEffect patterns with useSyncExternalStore. All changes maintain identical user-facing behavior.

Changes

1. useSyncExternalStore for SSR hydration handoff

Before: setTimeout → ReactDOM.flushSync(setColorMode(resolved)) → setColorMode(original) — 3 renders, forced sync flush, temporarily corrupted colorMode state. The previous commit simplified this to useState + useEffect, but still caused a post-hydration re-render.

After: useSyncExternalStore with getServerSnapshot reading the <script type="application/json"> tag during hydration, and getSnapshot resolving from client state after. Zero post-hydration re-renders. No effects.

Also removes the ReactDOM import entirely.

2. useSyncExternalStore for useSystemColorMode

Before: useState(getSystemColorMode) + useEffect subscribing to matchMedia changes — potential stale-then-update flicker between init and effect, and a redundant setSystemColorMode call to catch changes during the gap.

After: useSyncExternalStore(subscribeToSystemColorMode, getSystemColorMode, () => "day") — always current, no effect gap, no state.

3. Cached getServerHandoff

DOM read + JSON.parse is now cached per ID in a Map. The <script> tag content is static (written once during SSR), so subsequent calls return the cached result.

4. Memoized context value

The ThemeContext.Provider value was a new object literal every render, causing all consumers to re-render even when nothing changed. Now wrapped in useMemo.

Scenario analysis

Scenario	Before (original)	After
No SSR handoff (common)	Effect skips	No effect exists
SSR match	1+ extra renders via effect	0 extra renders (useSyncExternalStore handles transition)
SSR mismatch	3 renders via flushSync cascade	Synchronous correction, no cascade
System color mode changes	useEffect subscribes, potential flicker	useSyncExternalStore — always synchronous
matchMedia calls	1 per render (new MediaQueryList allocation)	1 per render (browser returns same cached object per spec)
Repeated parent renders	DOM read + JSON.parse every time	Cached after first call

Testing

All existing ThemeProvider tests pass (20 in @primer/react, 29 in @primer/styled-react).

[changeset-bot]

🦋 Changeset detected

Latest commit: e11672a

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Patch
@primer/styled-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.

[Copilot]

Pull request overview

This PR refactors ThemeProvider implementations (both @primer/react and @primer/styled-react) to simplify SSR color-mode handoff handling by removing the ReactDOM-based flushSync workaround and by making context value construction more stable.

Changes:

• Replace the server color-mode passthrough useRef + ReactDOM.flushSync workaround with a serverColorMode state that is cleared after hydration.
• Memoize the ThemeContext.Provider value via useMemo to reduce unnecessary context value churn.
• Remove the unused react-dom import from both ThemeProvider implementations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/styled-react/src/components/ThemeProvider.tsx	Aligns styled-react ThemeProvider SSR handoff logic with a state-based server passthrough and memoized context value.
packages/react/src/ThemeProvider.tsx	Updates core ThemeProvider SSR handoff logic to remove ReactDOM.flushSync usage and memoizes the context value.

Comments suppressed due to low confidence (1)

packages/react/src/ThemeProvider.tsx:88

• ThemeProvider has an existing unit test suite, but the preventSSRMismatch / server handoff path introduced here isn’t covered. Please add a test that simulates server handoff data (e.g., by mocking useId to a stable value and inserting a matching __PRIMER_DATA_<id>__ script element) and asserts that the provider initially uses the server-resolved color mode and then switches to client resolution after hydration.

  // Lazy initializer reads DOM + parses JSON once instead of every render
  const [serverColorMode, setServerColorMode] = React.useState<ColorMode | undefined>(
    () => getServerHandoff(uniqueDataId).resolvedServerColorMode,
  )

  const [colorMode, setColorMode] = useSyncedState(props.colorMode ?? fallbackColorMode ?? defaultColorMode)
  const [dayScheme, setDayScheme] = useSyncedState(props.dayScheme ?? fallbackDayScheme ?? defaultDayScheme)
  const [nightScheme, setNightScheme] = useSyncedState(props.nightScheme ?? fallbackNightScheme ?? defaultNightScheme)
  const systemColorMode = useSystemColorMode()
  const resolvedColorMode = serverColorMode ?? resolveColorMode(colorMode, systemColorMode)
  const colorScheme = chooseColorScheme(resolvedColorMode, dayScheme, nightScheme)
  const {resolvedTheme, resolvedColorScheme} = React.useMemo(
    () => applyColorScheme(theme, colorScheme),
    [theme, colorScheme],
  )

  // After hydration, clear the server passthrough so client-side color mode takes over
  React.useEffect(
    function clearServerPassthrough() {
      if (serverColorMode !== undefined) {
        setServerColorMode(undefined)
      }
    },
    [serverColorMode],
  )

[mattcosta7]

CI in gh/gh-ui wil fail until the current release merges and removes the unused feature flag

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/17032

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!