So it works for me!

What servlet container are you using?

Stephen Connolly how do i check?

It's jenkins 2.45 built via https://github.com/saltstack-formulas/jenkins-formula

I installed from scratch on this demo.

The only other thing is I'm using Jenkinsfile to define my parameters. Which means the first time it runs, it does not know about the parameters, after build #1 it knows.
If you look at the gif i attached, towards the end, any branches with a

/

dont have the option to 'build with parameters' because I can't initially build it.

Also: using this setup, no jobs allow me to build them. Slash or not, there is no "build" button for the job, so "Buidl with parameters" only shows up on parameterized builds. Is it an issue with state? It does not know about my parameters until it builds it, but i cannot build it because it doesn't have parameters?

So after chatting on IRC, this seems to be a confluence of different things:

1. It is somewhat confusing in the UI differentiating between a branch that has never been built and a dead branch. (Hint: currently the dead branches cannot be built but can be deleted... live branches cannot be deleted but can be built)
2. BuildWithParameters URL is not available until the Jenkinsfile has been loaded for the branch and the parameter definition applied

https://issues.jenkins-ci.org/browse/JENKINS-41927 created to help disambiguate orphaned items

I think this should let you go ahead and submit parameters. Ditto here. ParametersAction already refuses to set environment variables for undeclared parameters, so there should no risk in that. Similarly, this should be more lenient. Of course for undeclared parameters we do not know the type, so we could just create StringParameterValue in all cases.