perf(useIsMacOS): replace useState+useEffect with useSyncExternalStore

.changeset/perf-useismacOS-sync-external-store.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+perf(useIsMacOS): replace useState+useEffect with useSyncExternalStore to eliminate unnecessary re-render

packages/react/src/hooks/useIsMacOS.ts

@@ -1,19 +1,30 @@
 import {isMacOS as ssrUnsafeIsMacOS} from '@primer/behaviors/utils'
-import {useEffect, useState} from 'react'
-import {canUseDOM} from '../utils/environment'
+import {useSyncExternalStore} from 'react'
+
+// No-op. The platform never changes at runtime, so there is nothing to
+// subscribe to. Hoisted to avoid creating a new function on every call.
+const subscribe = () => () => {}
+
+// Safe default for SSR since we can't detect the platform on the server.
+const getServerSnapshot = () => false
 
 /**
- * SSR-safe hook for determining if the current platform is MacOS. When rendering
- * server-side, will default to non-MacOS and then re-render in an effect if the
- * client turns out to be a MacOS device.
+ * SSR-safe hook for determining if the current platform is MacOS.
+ *
+ * Uses `useSyncExternalStore` to read the platform value:
+ *
+ * - On the **client**, `ssrUnsafeIsMacOS` reads `navigator.userAgent` and
+ *   returns the real value immediately, with no extra render pass.
+ *
+ * - On the **server**, returns `false`. During hydration, if the snapshots
+ *   differ, React handles the mismatch internally in a single synchronous
+ *   pass, avoiding the layout shift that a deferred `useEffect` + `setState`
+ *   would cause.
+ *
+ * Previous implementation used `useState` + `useEffect`, which caused an
+ * unconditional second render on every mount (even on the client where the
+ * initial value was already correct).
  */
 export function useIsMacOS() {
-  const [isMacOS, setIsMacOS] = useState(() => (canUseDOM ? ssrUnsafeIsMacOS() : false))
-
-  useEffect(() => {
-    // eslint-disable-next-line react-hooks/set-state-in-effect
-    setIsMacOS(ssrUnsafeIsMacOS())
-  }, [])
-
-  return isMacOS
+  return useSyncExternalStore(subscribe, ssrUnsafeIsMacOS, getServerSnapshot)
 }