Java-side resource is not available if resource not referenced in a CSS file

[mshabarov]

Changing the style on java side, like below

getElement().getStyle().set("background-image",
                "url('theme/my-theme/images/background.jpg')");

does not work if the resource (background.jpg in our case) is not mentioned in any CSS file within /theme/my-theme/ folder. But it does work, if adding the reference to that resource (whatever the target element is), for example:

.some-target {
    background: url("./images/background.jpg");
}

It impacts both the dev and production mode.

[caalador]

Here we should not that we should only copy files that are not handled by webpack through the file-loader.
This is especially a challenge in development mode where the files are not copied to the disk, but are served by the running devServer.