[Edit] Refresh Library less aggressively

Update the root object (used to populate the Library pane)
less often, to avoid recreating the whole tree and thereby
causing the tree to collapse. WTD-788.

platform/commonUI/edit/src/controllers/EditPanesController.js

@@ -0,0 +1,47 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Supports the Library and Elements panes in Edit mode.
+         * @constructor
+         */
+        function EditPanesController($scope) {
+            var root;
+
+            // Update root object based on represented object
+            function updateRoot(domainObject) {
+                var context = domainObject &&
+                    domainObject.getCapability('context'),
+                    newRoot = context && context.getRoot(),
+                    oldId = root && root.getId(),
+                    newId = newRoot && newRoot.getId();
+
+                // Only update if this has actually changed,
+                // to avoid excessive refreshing.
+                if (oldId !== newId) {
+                    root = newRoot;
+                }
+            }
+
+            // Update root when represented object changes
+            $scope.$watch('domainObject', updateRoot);
+
+            return {
+                /**
+                 * Get the root-level domain object, as reported by the
+                 * represented domain object.
+                 * @returns {DomainObject} the root object
+                 */
+                getRoot: function () {
+                    return root;
+                }
+            };
+        }
+
+        return EditPanesController;
+    }
+);