[JENKINS-25455] Avoid zero height of CodeMirror textarea

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

 Behaviour.specify("TEXTAREA.codemirror", 'textarea', 0, function(e) {
+        //ensure, that textarea is visible, when obtaining its height, see JENKINS-25455
+        function getTextareaHeight() {
+            var p = e.parentNode.parentNode; //first parent is CodeMirror div, second is actual element which needs to be visible
+            var display = p.style.display; 
+            p.style.display = "";
             var h = e.clientHeight;
+            p.style.display = display;
+            return h;
+        }
+        
+        var h = e.clientHeight || getTextareaHeight();
         var config = e.getAttribute("codemirror-config") || "";
         config = eval('({'+config+'})');
         var codemirror = CodeMirror.fromTextArea(e,config);
@@ -20,6 +30,12 @@ Behaviour.specify("TEXTAREA.codemirror", 'textarea', 0, function(e) {
 			e.value = codemirror.getValue()
 		})
         }
+		
+        //refresh CM when there are some layout updates
+        function refreshCM() {
+            codemirror.refresh();
+        }
+        layoutUpdateCallback.add(refreshCM);
     });
 
 Behaviour.specify("DIV.textarea-preview-container", 'textarea', 100, function (e) {