`window` is injected as an `object` in webworker scripts instead of `undefined`

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/boring-hill-2w8x38?file=%2Fapp%2Fworker.ts%3A5%2C44

To Reproduce

1. Start the application in dev
2. Click the "Send to worker" button
3. Inspect the console

Current vs. Expected behavior

Until next@14.0.5-canary.20, typeof window returned undefined inside a webworker as expected. However since next@14.0.5-canary.20, typeof window returns object but this object cannot be accessed. Therefore, as it stands, third party libraries making use of window via an undefined test cannot work anymore in webworkers in Nextjs.

The most notable example if Three.js which includes this snippet at the end of the library -

if ( typeof window !== 'undefined' ) {

	if ( window.__THREE__ ) {

		console.warn( 'WARNING: Multiple instances of Three.js being imported.' );

	} else {

		window.__THREE__ = REVISION;

	}

}

Provide environment information

All

Which area(s) are affected? (Select all that apply)

SWC transpilation

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local), Other (Deployed)

Additional context

This bug was first introduced in next@14.0.5-canary.20

Start with the linked CodeSandbox reproduction in app/worker.ts and verify the behavior in the listed development and production stages. Then trace the SWC transpilation involved in webworker scripts. Done means typeof window is undefined again, allowing the shown Three.js guard to work without accessing the injected object.