List: Clean up observers that will never be used; refs #642

List.js

@@ -442,14 +442,15 @@ function(kernel, declare, listen, has, miscUtil, TouchScroll, hasClass, put){
 			options = options || {};
 			var self = this,
 				start = options.start || 0,
-				row, rows, container;
+				observers = this.observers,
+				row, rows, container, observerIndex;
 
 			if(!beforeNode){
 				this._lastCollection = results;
 			}
 			if(results.observe){
 				// observe the results for changes
-				var observerIndex = this.observers.push(results.observe(function(object, from, to){
+				observerIndex = observers.push(results.observe(function(object, from, to){
 					var firstRow, nextNode, parentNode;
 					// a change in the data took place
 					if(from > -1 && rows[from]){
@@ -503,34 +504,50 @@ function(kernel, declare, listen, has, miscUtil, TouchScroll, hasClass, put){
 					self._onNotification(rows, object, from, to);
 				}, true)) - 1;
 			}
-			var rowsFragment = document.createDocumentFragment();
-			// now render the results
-			if(results.map){
-				rows = results.map(mapEach, console.error);
-				if(rows.then){
-					return rows.then(whenDone);
-				}
-			}else{
-				rows = [];
-				for(var i = 0, l = results.length; i < l; i++){
-					rows[i] = mapEach(results[i]);
-				}
-			}
-			var lastRow;
+			var rowsFragment = document.createDocumentFragment(),
+				lastRow;
+
 			function mapEach(object){
 				lastRow = self.insertRow(object, rowsFragment, null, start++, options);
 				lastRow.observerIndex = observerIndex;
 				return lastRow;
 			}
+			function whenError(error){
+				if(typeof observerIndex !== "undefined"){
+					observers[observerIndex].cancel();
+					observers[observerIndex] = 0;
+				}
+				if(error){
+					throw error;
+				}
+			}
 			function whenDone(resolvedRows){
 				container = beforeNode ? beforeNode.parentNode : self.contentNode;
-				if(container){
+				if(container && container.parentNode && resolvedRows.length){
 					container.insertBefore(rowsFragment, beforeNode || null);
 					lastRow = resolvedRows[resolvedRows.length - 1];
 					lastRow && self.adjustRowIndices(lastRow);
+				}else if(observers[observerIndex]){
+					// Remove the observer and don't bother inserting;
+					// rows are already out of view or there were none to track
+					whenError();
 				}
 				return (rows = resolvedRows);
 			}
+
+			// now render the results
+			if(results.map){
+				rows = results.map(mapEach, console.error);
+				if(rows.then){
+					return rows.then(whenDone, whenError);
+				}
+			}else{
+				rows = [];
+				for(var i = 0, l = results.length; i < l; i++){
+					rows[i] = mapEach(results[i]);
+				}
+			}
+
 			return whenDone(rows);
 		},