fix(react-virtual): defer flushSync to microtask for React 19 compatibility

[channyeintun]

React 19 throws a warning when flushSync is called from inside a lifecycle method (useLayoutEffect). This change defers the flushSync call to a microtask using queueMicrotask(), allowing React to complete its current render cycle before forcing the synchronous update.

Fixes #1094

Fixes the warning: flushSync was called from inside a lifecycle method. React cannot flush when React is already rendering.

🎯 Changes

Problem

When using @tanstack/react-virtual with React 19, users see the following warning in the console:

flushSync was called from inside a lifecycle method. React cannot flush when React is already rendering. Consider moving this call to a scheduler task or micro task.

This occurs because flushSync(rerender) is called inside the onChange callback, which can be triggered during useLayoutEffect via instance._willUpdate().

Solution

Wrapped the flushSync call in queueMicrotask() to defer it to the next microtask:

- flushSync(rerender)
+ queueMicrotask(() => flushSync(rerender))

This ensures React completes its current render cycle before the synchronous update is forced, while still maintaining the synchronous update semantics for the virtualizer.

Files Changed

• packages/react-virtual/src/index.tsx - Wrap flushSync in queueMicrotask
• packages/react-virtual/tests/index.test.tsx - Update scroll test to be async

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

🦋 Changeset detected

Latest commit: e888816

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

This PR includes changesets to release 1 package

Name	Type
@tanstack/react-virtual	Patch

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