Add a small delay after an xhr request is complete before submit_disabled...

Add a small delay after an xhr request is complete before submit_disabled class is removed from the body.  This helps to reduce the flicker users see when email addresses are being checked and there are consecutive XHR requests.

issue #1898

resources/static/common/js/modules/xhr_disable_form.js

@@ -15,8 +15,17 @@ BrowserID.Modules.XHRDisableForm = (function() {
 
       self.subscribe("xhr_start",
         dom.addClass.curry("body", "submit_disabled"));
-      self.subscribe("xhr_complete",
-        dom.removeClass.curry("body", "submit_disabled"));
+      self.subscribe("xhr_complete", function() {
+        // Add a small delay between the time the XHR is complete and when the
+        // submit_disabled class is actually removed.  This helps reduce the
+        // amount of flicker the user sees if one XHR request completes and
+        // another one starts immediately afterwards.
+        // See https://github.com/mozilla/browserid/issues/1898
+        setTimeout(function() {
+          dom.removeClass("body", "submit_disabled");
+          self.publish("submit_enabled");
+        }, 100);
+      });
 
       sc.start.call(self, options);
     }

resources/static/test/cases/common/js/modules/xhr_disable_form.js

@@ -29,13 +29,19 @@
     }
   });
 
-  test("xhr_start adds 'submit_disabled' to class, xhr_complete removes it", function() {
+  asyncTest("xhr_start adds 'submit_disabled' to class, xhr_complete removes it", function() {
     var body = $("body");
 
     mediator.publish("xhr_start");
     equal(body.hasClass("submit_disabled"), true, "xhr_start adds submit_disabled");
 
+    // submit_disabled is removed after a small delay so that if consecutive
+    // XHR requests happen, there is no button flicker. See issue #1898
+    // - https://github.com/mozilla/browserid/issues/1898
+    mediator.subscribe("submit_enabled", function() {
+      equal(body.hasClass("submit_disabled"), false, "xhr_complete removes submit_disabled");
+      start();
+    });
     mediator.publish("xhr_complete");
-    equal(body.hasClass("submit_disabled"), false, "xhr_complete removes submit_disabled");
   });
 }());