If you're migrating tests to React 19, this gives you the exact before/after patterns for the breaking changes. It covers the act() import move from react-dom/test-utils to react, replacing the removed Simulate API with fireEvent, and the StrictMode call count adjustments since effects no longer double-invoke in development. The priority order is smart: fix act() imports first since everything else depends on it, then work through Simulate, then tackle the call count assertions. The StrictMode section is honest about measuring actual counts instead of guessing, which will save you from chasing phantom bugs when your spies expect 2 calls but now get 1.
npx skills add https://github.com/github/awesome-copilot --skill react19-test-patterns