Frontend Files in V14 Multi Project setup

How do I need to handle frontend resources that are from another project?

I have problems with upgrading a V13 Project (“PROJ_A”) of mine. My Issue seems to come from the fact that this project has a dependency to another project (“PROJ_B”) of mine, where I define my own Implementations of Vaadin Components and static Helper Classes, to be reused in other vaadin projects.

PROJ_B has a Class CloseButton which extends Button, and is annotated with @CssImport("./styles/components/closeButton.css"). This css file ist stored in [project-root] /frontend/styles/components/closeButton.css.

When I go and install and run the PROJ_B I see in the file [project-root] /target/frontend/generated-flow-imports.js it added two lines about the closeButton.css:

import $css_0 from 'Frontend/styles/components/closeButton.css';

Looking at a TestView that I implemented in PROJ_B, it works and the CloseButton can be displayed.

But now I would like to use that CloseButton in another Project, PROJ_A. I have followed the Migration Tutorial for this project and everything seems to work, if I don’t use the CloseButton anywhere, so the migration part seems to be finished. But when I now include a CloseButton in a View, running the PROJ_A will fail with an IllegalStateException, saying that it could not find the css file styles/components/closeButton.css.

java.lang.IllegalStateException: Failed to update the Flow imports file 'C:\pathToProject\PROJ_A\target\frontend\generated-flow-imports.js'
at ....
Caused by: java.lang.IllegalStateException: 
   Failed to find the following css files in the `node_modules` or `/frontend` tree:
       - styles/components/closeButton.css
   Check that they exist or are installed.

As soon as I manually copy that css file from PROJ_B to PROJ_A, it works. But I can’t believe that this is the intended way to go. I would need to include any frontend files of all third-party add-ons etc manually in my project? I assumed it should work without doing that. Can anybody clear this up for me? Thank you

Update: I was able to make it work, but I’m not 100% sure what exactly fixed it. I assume it is the fact that I moved the css file from [project-root] /frontend/styles/components/closeButton.css to [project-root] /src/main/resources/META_INF/resources/frontend/styles/components/closeButton.css .[This Stackoverflow post]
( by @Tazavoo helped me very much.