I have identified two issues related with this behaviour with textarea fields and codemirror
Too many String Parameters
In a job with more than 30 String parameters, each parameter include a textarea field with HTML preview. As workaround, you can set the "Markup formatter" to "Plain text" in "Configure global security".
textarea resized (shell script,...)
the response times are too long when trying to configure a job with resized textarea fields.
Maybe the bug was introduced in https://issues.jenkins-ci.org/browse/JENKINS-25455 over this change https://github.com/jenkinsci/jenkins/commit/e9aeaf11b7171aed80381e8f66c5fb7056960141
and seems these concrete lines in textarea.js are involved in the bug: