Here we're sitting on the original branch:

✔ ~/Documents/CloudBees/Work/Declarative/parser-rewrite/ghe/declarative-parser-rewrite [failFast-gets-deleted|✔] 
15:58 $ git branch | grep '*'
* failFast-gets-deleted

Now look for the line in the Jenkinsfile:

15:50 $ cat Jenkinsfile | grep Fast
                failFast: false

Make the edit in the editor, and save to the new branch. This is steps 3-4 in the steps to recreate. Now, do a git pull to pull down the branch you created with the editor. Mine was called new-parallel-name:

✔ ~/Documents/CloudBees/Work/Declarative/parser-rewrite/ghe/declarative-parser-rewrite [failFast-gets-deleted|✔] 
15:51 $ git pull
Username for 'https://github.beescloud.com': dev1
Password for 'https://dev1@github.beescloud.com': 
remote: Counting objects: 3, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.beescloud.com/dev1/declarative-parser-rewrite
 * [new branch]      new-parallel-name -> origin/new-parallel-name
Already up-to-date.

Do a git checkout new-parallel-name to check out the branch you created in the editor:

✔ ~/Documents/CloudBees/Work/Declarative/parser-rewrite/ghe/declarative-parser-rewrite [failFast-gets-deleted|✔] 
16:01 $ git checkout new-parallel-name
Branch new-parallel-name set up to track remote branch new-parallel-name from origin.
Switched to a new branch 'new-parallel-name'

And finally, search this Jenkinsfile for the failFast: none line, and note that it's gone:

✔ ~/Documents/CloudBees/Work/Declarative/parser-rewrite/ghe/declarative-parser-rewrite [new-parallel-name|✔] 
16:01 $ !cat
cat Jenkinsfile | grep Fast

If you look closely at the two Jenkinsfiles, notice that there's a blank line where the failFast: false used to be.

Karl Shultz good find, we need to handle this properly, but it will be output in the 'new style' by the editor (I'm not sure how to do it offhand, need guidance from Andrew Bayer)

For what it's worth, I tried a simpler case, which only uses the "old style" parallel branches. Sample Jenkinsfile:

pipeline {
    agent any
    stages {
        stage('Parallel Steps As Before') {
            steps {
                parallel firstBranch: {
                    node('master') {
                        build 'ten-parallels'
                    }
                },
                secondBranch: {
                    node('master') {
                        build 'ten-parallels'
                    }
                },
                failFast: false
            }
        }
    }
}

Then I used the editor to change the name of Parallel Steps As Before to Old Style Parallel. The failFast: false line was still removed from the Jenkinsfile, with a blank line put in its place. It's easy to miss that blank line if you don't know to look for it, but it's there.

pipeline {
  agent any
  stages {
    stage('Old Style Parallel') {
      parallel {
        stage('firstBranch') {
          steps {
            node(label: 'master') {
              build 'ten-parallels'
            }

          }
        }
        stage('secondBranch') {
          steps {
            node(label: 'master') {
              build 'ten-parallels'
            }

          }
        }
      }
    }
  }
}

Assigning to Andrew Bayer as it sounds like there is no way to support this in "new-style" parallel at this time; probably the editor would just work at that point, but perhaps not, at which point just assign this back to me.

https://github.com/jenkinsci/pipeline-model-definition-plugin/pull/201 is up - I think it does the trick.

Code changed in jenkins
User: Andrew Bayer
Path:
pipeline-model-api/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/ast/ModelASTStage.java
pipeline-model-definition/src/main/groovy/org/jenkinsci/plugins/pipeline/modeldefinition/model/Stage.groovy
pipeline-model-definition/src/main/groovy/org/jenkinsci/plugins/pipeline/modeldefinition/parser/JSONParser.groovy
pipeline-model-definition/src/main/groovy/org/jenkinsci/plugins/pipeline/modeldefinition/parser/ModelParser.groovy
pipeline-model-definition/src/main/groovy/org/jenkinsci/plugins/pipeline/modeldefinition/parser/RuntimeASTTransformer.groovy
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/AbstractModelDefTest.java
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/BasicModelDefTest.java
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/parser/ExecuteConvertedTest.java
pipeline-model-definition/src/test/resources/json/parallelStagesFailFast.json
pipeline-model-definition/src/test/resources/parallelStagesFailFast.groovy
http://jenkins-ci.org/commit/pipeline-model-definition-plugin/1a7baae6efcc0358a0ff2ec46e8141cafcee5f5c
Log:
[FIXED JENKINS-47109] stage-level failFast w/ parallel stages

Might want to also reject if you use failFast in a non-parallel-parent
stage, but it's pretty much harmless, so...eh.