tanstackRouter.test.ts fails on upcoming-v16.3.0 with Node 20 (React.act not available)

[AbanoubGhadban]

Problem

packages/react-on-rails-pro/tests/tanstackRouter.test.ts fails on the upcoming-v16.3.0 branch when run with Node 20:

React.act is not available — React 18.3+ or 19+ is required

The test passes on main with Node 20 (verified on branch jg/concurrent-worktree-ports).

CI evidence

• Fails: build (20) on fix/rsc-streaming-flush (based on upcoming-v16.3.0)
• Passes: build (20) on jg/concurrent-worktree-ports (based on main)

Likely cause

A dependency or configuration difference between upcoming-v16.3.0 and main that affects which React version is resolved for Node 20 in the test environment. The test requires React.act which is only available in React 18.3+.

Branch

This issue exists on upcoming-v16.3.0, not on main.