[FIXED JENKINS-18585]

The earlier fix by @ssogabe broke the test. This fix corrects the
problem properly.

The reason this error happens with some plugins is that if there's an
input field whose name is "on", then even after prototype.js extends
an element object, "form.on" will refer to the input element, not the
prototype.js function to add an event handler.

changelog.html

@@ -63,6 +63,9 @@ Upcoming changes</a>
 <div id="rc" style="display:none;"><!--=BEGIN=-->
 <h3><a name=v1.522>What's new in 1.522</a> <!--=DATE=--></h3>
 <ul class=image>
+  <li class=bug>
+    Fixed a regression in the config form with some plugins
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-18585">issue 18585</a>)
   <li class=bug>
     Fixed a dead lock in the <tt>Project</tt> class and improved the signature of the persisted XML form a bit.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-18589">issue 18589</a>)

core/src/main/resources/lib/form/textarea/textarea.js

@@ -15,7 +15,7 @@ Behaviour.specify("TEXTAREA.codemirror", 'textarea', 0, function(e) {
         scroller.style.height = h+"px";
 
         // the form needs to be populated before the "Apply" button
-        YAHOO.util.Event.on(e.up('form'), "jenkins:apply", function() {
+    Element.on(e.up('form'),"jenkins:apply", function() {
             e.value = codemirror.getValue()
         })
     });