Why do unit tests that import the core module produce the following warning:
An instance of PageScrollService already exists, usually including one provider should be enough, so double check

Is the service supposed to be providedIn: 'root' ? You have to import the module in order for the ngxps_config injection token to be set, and the module provides the service.

Some reference implementations of how to use this in combination with e.g. jQuery or more advanced libraries like smooth-scroll-into-view-if-needed would be handy.

Like this:

examples/
 - basic.js
 - jquery.js
 - advanced.js