Perf: Static children count for wasted time

src/test/ReactDefaultPerf.js

@@ -205,6 +205,9 @@ var ReactDefaultPerf = {
           var id = args[0];
           if (moduleName === 'EventPluginHub') {
             id = id._rootNodeID;
+          } else if (fnName === 'replaceNodeWithMarkup') {
+            // Old node is already unmounted; can't get its instance
+            id = ReactDOMComponentTree.getInstanceFromNode(args[1].node)._rootNodeID;
           } else if (typeof id === 'object') {
             id = ReactDOMComponentTree.getInstanceFromNode(args[0])._rootNodeID;
           }

src/test/__tests__/ReactDefaultPerf-test.js

@@ -36,13 +36,13 @@ describe('ReactDefaultPerf', function() {
 
     App = React.createClass({
       render: function() {
-        return <div><Box /><Box /></div>;
+        return <div><Box /><Box flip={this.props.flipSecond} /></div>;
       },
     });
 
     Box = React.createClass({
       render: function() {
-        return <div><input /></div>;
+        return <div key={!!this.props.flip}><input /></div>;
       },
     });
 
@@ -84,6 +84,28 @@ describe('ReactDefaultPerf', function() {
     /*eslint-enable dot-notation */
   });
 
+  it('should count no-op update in child as waste', function() {
+    var container = document.createElement('div');
+    ReactDOM.render(<App />, container);
+
+    // Here, we add a Box -- two of the <Box /> updates are wasted time (but the
+    // addition of the third is not)
+    var measurements = measure(() => {
+      ReactDOM.render(<App flipSecond={true} />, container);
+    });
+
+    var summary = ReactDefaultPerf.getMeasurementsSummaryMap(measurements);
+    expect(summary.length).toBe(1);
+
+    /*eslint-disable dot-notation */
+
+    expect(summary[0]['Owner > component']).toBe('App > Box');
+    expect(summary[0]['Wasted time (ms)']).not.toBe(0);
+    expect(summary[0]['Instances']).toBe(1);
+
+    /*eslint-enable dot-notation */
+  });
+
   function expectNoWaste(fn) {
     var measurements = measure(fn);
     var summary = ReactDefaultPerf.getMeasurementsSummaryMap(measurements);