1. visit https://ci.jenkins.io/pipeline-syntax/
2. 
   sample step: library;
3. 
   retrieval method: modern scm

I'm not sure where the fault is/what code is operative.

The client might be:

https://github.com/jenkinsci/github-branch-source-plugin/blob/76bfd3efcbd68ec6ceffa0c4860a55497ea45219/src/main/resources/org/jenkinsci/plugins/github_branch_source/GitHubSCMNavigator/config.jelly#L4

I'm wondering if the problem is a bad interaction between some magic hoisting and jenkinsci/jenkins: core/src/main/resources/lib/form/invisibleEntry.jelly

Found w/ 2.150.3 (ci.j.o) and 2.166 (localhost)