perf: defer useAnchoredPosition setState on initial mount

.changeset/defer-anchored-position-mount.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Defer `useAnchoredPosition` initial mount setState from useLayoutEffect to useEffect when overlay is closed, eliminating unnecessary cascading re-renders that block paint.

packages/react/src/hooks/__tests__/useAnchoredPosition.test.tsx

@@ -35,6 +35,47 @@ it('should should return a position', async () => {
   })
 })
 
+it('should defer initial updatePosition to useEffect when overlay is closed on mount', async () => {
+  // When no floating element is present (overlay closed), the initial
+  // updatePosition call should be deferred from useLayoutEffect to useEffect.
+  // We verify this by checking that onPositionChange has NOT been called by
+  // the time the component's own useLayoutEffect runs (which fires after the
+  // hook's useLayoutEffect in declaration order).
+  const onPositionChange = vi.fn()
+  const layoutPhaseCheck = vi.fn()
+
+  const ClosedOverlayComponent = ({
+    onPositionChangeProp,
+    onLayoutEffect,
+  }: {
+    onPositionChangeProp: typeof onPositionChange
+    onLayoutEffect: (calledDuringLayout: boolean) => void
+  }) => {
+    const floatingElementRef = React.useRef<HTMLDivElement>(null)
+    const anchorElementRef = React.useRef<HTMLDivElement>(null)
+    useAnchoredPosition({floatingElementRef, anchorElementRef, onPositionChange: onPositionChangeProp})
+
+    // This layout effect runs after the hook's layout effects (declaration order).
+    // With the fix, onPositionChange should NOT have been called yet because
+    // the initial updatePosition is deferred to useEffect.
+    React.useLayoutEffect(() => {
+      onLayoutEffect(onPositionChangeProp.mock.calls.length > 0)
+    }, [onPositionChangeProp, onLayoutEffect])
+
+    return <div />
+  }
+
+  render(<ClosedOverlayComponent onPositionChangeProp={onPositionChange} onLayoutEffect={layoutPhaseCheck} />)
+
+  // onPositionChange should not have fired during the layout phase
+  expect(layoutPhaseCheck).toHaveBeenCalledWith(false)
+
+  // After effects run, onPositionChange should have been called with undefined
+  await waitFor(() => {
+    expect(onPositionChange).toHaveBeenCalledWith(undefined)
+  })
+})
+
 describe('scroll recalculation', () => {
   it('should recalculate position when window scrolls', async () => {
     const cb = vi.fn()

packages/react/src/hooks/useAnchoredPosition.ts

@@ -116,7 +116,26 @@ export function useAnchoredPosition(
     savedOnPositionChange.current = settings?.onPositionChange
   }, [settings?.onPositionChange])
 
-  useLayoutEffect(updatePosition, [updatePosition])
+  // Defer the first updatePosition to useEffect when the overlay is closed on
+  // mount, avoiding paint-blocking cascading setState. If the overlay is already
+  // open on mount, run synchronously in useLayoutEffect to prevent a flash.
+  // After mount, all updates use useLayoutEffect.
+  const hasMountedRef = React.useRef(false)
+  useLayoutEffect(() => {
+    if (hasMountedRef.current) {
+      updatePosition()
+    } else if (floatingElementRef.current instanceof Element && anchorElementRef.current instanceof Element) {
+      hasMountedRef.current = true
+      updatePosition()
+    }
+  }, [updatePosition, floatingElementRef, anchorElementRef])
+
+  React.useEffect(() => {
+    if (!hasMountedRef.current) {
+      hasMountedRef.current = true
+      updatePosition()
+    }
+  }, [updatePosition])
 
   useResizeObserver(updatePosition) // watches for changes in window size
   useResizeObserver(updatePosition, floatingElementRef as React.RefObject<HTMLElement | null>) // watches for changes in floating element size