Vitest 4 recently came out, and it's now extremely sensitive to when you can use arrow functions vs normal functions. The linter rule complexity.useArrowFunction needs to be turned to off when using Vite 4 so it does not convert the normal functions to arrow functions.

https://main.vitest.dev/guide/migration.html#spyon-and-fn-support-constructors

Not sure what an appropriate fix for this would be...