Root provided services: angular.dev/guide/di/dep...

Singleton Pattern: en.wikipedia.org/wiki/Singlet...

I'm glad you got it working! Even when well rested unit tests are ridiculously hard sometimes.

If `routerLink` is on an anchor tag, it subscribes to `router.events` in the constructor. It does not do this for a button. Is the test providing a router?
github.com/angular/angu...

This is switching from Angular modules + components to just Angular components. It would be analogous to migrating from class-based to functional components in React, maybe? Lots of effort, I never have to explain them to newer devs, I can remove the entire concept from my brain.