Java / Vaadin only Addon with CSSImport breaks starting of Spring Boot App

I created a small Vaadin Addon with the Vaadin Addon Starter. The Addon shows a process line.
See this Git-Repo for the source-code https://github.com/berndklb/vaadin-progress-line

@CssImport("./styles/progress-line.css")
public class ProgressLine extends Composite<FlexLayout> implements HasItems<String>  
{
	private LinkedList<String> items;
	private String active;
	private LinkedList<ItemLabel> itemLabels = new LinkedList<>();
	private FlexLayout progressLayout;
	private Span lblHeadline;
	
	public ProgressLine(String label, List<String> items, String active) {
		super();
		this.items = new LinkedList<>(items);
		this.active = active;
		
		this.buildUI(label);
	}
...
}

If I add the maven dependency to my Vaadin Application (https://github.com/berndklb/vaadin-bugs/tree/bug/addon_css_import) and tries to start the Spring Boot Project, I get the following exception:

Failed to find the following css files in the `node_modules` or `D:\Repos\my-bug-project\frontend` directory tree:
      - ./styles/progress-line.css
  Check that they exist or are installed. If you use a custom directory for your resource files instead of the default `frontend` folder then make sure it's correctly configured (e.g. set 'vaadin.frontend.frontend.folder' property)

The progress-line.css can not be found, because it is inside the addon jar. If I remove the @CSSImport from ProgressLine.java I can use the Addon in the webapplication.
So what is the way to create an addon and use it another webapp? Is there any documentation about this?

There is a [post on Stackoverflow]
(https://stackoverflow.com/a/57553974/3441504) that you can use to find out where frontend and static files should go in which situation.

For Addons, the frontend folder should be in {project-root}/src/main/resources/META-INF/resources/frontend/

Edit: I found another ‘documentation’ about this, in your own github of the progress-line project in the readme. It’s still the default readme from the addon-starter featuring the paper-slider, but it mentions this:

Development instructions
JavaScript modules can either be published as an NPM package or be kept as local files in your project. The local JavaScript modules should be put in src/main/resources/META-INF/frontend so that they are automatically found and used in the using application.

(This path has one less resources folder in it than the path from the stackoverflow answer. I don’t know if this works as well, but I have successfully built my own add on project and included it in an application project, using the path {project-root}/src/main/resources/META-INF/resources/frontend/ for my css and js files)