Pipeline library functions ideally would show up "like" steps in the blue ocean editor.
This may require additional type information so the REST API can furnish the editor with the meta data as needed. If the functions were delivered via a plugin, say like https://github.com/jenkinsci/simple-build-for-pipeline-plugin - then there is a possibility for an interface to be used that can be discovered to provide the meta data needed to make the function calls "look like" a step (ie blue ocean can lookup function extension metadata, and service it up for the editor). Libraries don't have type information now, so a mechanism like this would be acceptable for those that want to have the metadata/editor support.
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The only thing holding us back from starting to use the pipeline editor is an inability to include our shared groovy library steps. I'm not sure the level of effort that would be required to have them in the pipeline editor, but at this point a large chunk of our Jenkinsfiles are made up of calls to our shared libraries, and we currently have to ask people to avoid using the editor because they can't even see the documentation for those libraries while in blue ocean.