Culprit:
(1) - MatrixProject/configure-entries.jelly#L52

Reproduction steps:

• Install Matrix Project or clone https://github.com/jenkinsci/matrix-project-plugin and then mvn hpi:run
• Create a Multi-configuration project
• On the "Advanced Project Options" click on the "Advanced..." button
• Add a String in the "display name" field
• Click somewhere else in the page
• It should send a Post request to the checkDisplayName endpoint with your string as parameter

Screenshot attached about where the feature is displayed.

Proposal
https://www.jenkins.io/doc/developer/security/csp/#legacy-javascript-checkurl-validation

Testing notes
 - Ensure to reproduce the feature before any change
 - Ensure that you reproduce the feature after you have made the change