After upgrading pipeline-model-definition 1.38 to 1.39, I started seeing strange behavior in one declarative pipeline. Unfortunately it's a complicated pipeline and I haven't had time to come up with a minimal case to reproduce the issue.

In some situations, the Blue Ocean UI doesn't show progress. It appears as if the build hasn't started yet, even though it has (i.e. go to the console output in the classic UI, and the build is running).  When the build gets to an input prompt, the header bar of Blue Ocean turns the "input prompt blue", but no prompt is visible, and no progress is still shown. It's necessary to navigate to the "classic" UI to respond to the prompt.

The pipeline has a call to `input()` in the steps of its first stage. I don't know if this is relevant. I haven't seen the problem in our other pipelines, or even in an obviously reproducible way with the pipeline that does exhibit the issue. Downgrading to pipeline-model-definition:1.38 fixes the issue. Sorry for the vague report. I had to do the downgrade quickly to unblock my teammates, and didn't have time to investigate thoroughly.