hook this up in behaviour

core/src/main/resources/lib/form/dropdownList.jelly

@@ -46,7 +46,7 @@ THE SOFTWARE.
         </td>
         <td class="setting-main">
             <!-- create drop-down list -->
-            <select class="setting-input dropdownList" onchange="updateDropDownList(this)">
+            <select class="setting-input dropdownList">
                 <j:set var="dropdownListMode" value="createSelectField"/>
                 <d:invokeBody/>
             </select>

war/src/main/webapp/scripts/hudson-behavior.js

@@ -957,6 +957,8 @@ var hudsonRules = {
             start = findFollowingTR(end, 'dropdownList-start');
         }
 
+        e.onchange = function () { updateDropDownList(e); };
+
         updateDropDownList(e);
     },