When wrapping parallel steps in a retry block, if you attempt to abort the run after a stage has failed, it will continue retrying instead of aborting.  Jenkins will also mark the build result as aborted, even though it continued with the retry and failed.  Because of this the stop button in Blue Ocean will be grayed out, until the page is refreshed.  And the only way to get the pipeline to finally abort, is to either abort it before a failure happens, or manually abort each retry.

node {    
    stage('one') { 
       sleep 5 
    }
    stage('two') {
        retry(3) {
            parallel slowTask1: {
                sleep 30
            }, slowTask2: {
                sleep 30
            }, failingTask: {
                sh 'exit 1'
            },
            failFast: false
        }
    }
}