Give context aware message if markup reuse fails

The context is an extra message with comparison of where server and
client markup started to differ.

src/browser/ui/ReactMount.js

@@ -54,6 +54,22 @@ if (__DEV__) {
 // Used to store breadth-first search state in findComponentRoot.
 var findComponentRootReusableArray = [];
 
+/**
+ * Finds the index of the first character
+ * that's not common between the two given strings.
+ *
+ * @return {number} the index of the character where the strings diverge
+ */
+function firstDifferenceIndex(string1, string2) {
+  var minLen = Math.min(string1.length, string2.length);
+  for (var i = 0; i < minLen; i++) {
+    if (string1[i] !== string2[i]) {
+      return i;
+    }
+  }
+  return string1.length === string2.length ? -1 : minLen;
+}
+
 /**
  * @param {DOMElement} container DOM element that may contain a React component.
  * @return {?string} A "reactRoot" ID, if a React component is rendered.
@@ -719,11 +735,26 @@ var ReactMount = {
     );
 
     if (shouldReuseMarkup) {
-      if (ReactMarkupChecksum.canReuseMarkup(
-        markup,
-        getReactRootElementInContainer(container))) {
+      var rootElement = getReactRootElementInContainer(container);
+      if (ReactMarkupChecksum.canReuseMarkup(markup, rootElement)) {
         return;
       } else {
+        var checksum = rootElement.getAttribute(
+          ReactMarkupChecksum.CHECKSUM_ATTR_NAME
+        );
+        rootElement.removeAttribute(ReactMarkupChecksum.CHECKSUM_ATTR_NAME);
+
+        var rootMarkup = rootElement.outerHTML;
+        rootElement.setAttribute(
+          ReactMarkupChecksum.CHECKSUM_ATTR_NAME,
+          checksum
+        );
+
+        var diffIndex = firstDifferenceIndex(markup, rootMarkup);
+        var difference = ' (client) ' +
+          markup.substring(diffIndex - 20, diffIndex + 20) +
+          '\n (server) ' + rootMarkup.substring(diffIndex - 20, diffIndex + 20);
+
         invariant(
           container.nodeType !== DOC_NODE_TYPE,
           'You\'re trying to render a component to the document using ' +
@@ -733,7 +764,8 @@ var ReactMount = {
           'methods are impure. React cannot handle this case due to ' +
           'cross-browser quirks by rendering at the document root. You ' +
           'should look for environment dependent code in your components ' +
-          'and ensure the props are the same client and server side.'
+          'and ensure the props are the same client and server side:\n' +
+          difference
         );
 
         if (__DEV__) {
@@ -745,7 +777,7 @@ var ReactMount = {
             'new markup to compensate which works but you have lost many ' +
             'of the benefits of server rendering. Instead, figure out ' +
             'why the markup being generated is different on the client ' +
-            'or server.'
+            'or server:\n' + difference
           );
         }
       }

src/browser/ui/__tests__/ReactRenderDocument-test.js

@@ -210,7 +210,9 @@ describe('rendering React components at document', function() {
       'are impure. React cannot handle this case due to cross-browser ' +
       'quirks by rendering at the document root. You should look for ' +
       'environment dependent code in your components and ensure ' +
-      'the props are the same client and server side.'
+      'the props are the same client and server side:\n' +
+      ' (client) data-reactid=".0.1">Hello world</body></\n' +
+      ' (server) data-reactid=".0.1">Goodbye world</body>'
     );
   });