if you try posting a reply and are looking at a different topic, pop this window up

app/assets/javascripts/discourse/controllers/composer_controller.js

@@ -27,11 +27,56 @@ Discourse.ComposerController = Discourse.Controller.extend({
     }
   },
 
-  save: function() {
+  save: function(force) {
     var composer,
-      _this = this;
+      _this = this,
+      topic,
+      message,
+      buttons;
+
     composer = this.get('content');
     composer.set('disableDrafts', true);
+
+    // for now handle a very narrow use case
+    // if we are replying to a topic AND not on the topic pop the window up
+
+    if(!force && composer.get('replyingToTopic')) {
+      topic = this.get('topic');
+      if (!topic || topic.get('id') !== composer.get('topic.id'))
+      {
+        message = Em.String.i18n("composer.posting_not_on_topic", {title: this.get('content.topic.title')});
+
+        buttons = [{
+          "label": Em.String.i18n("composer.cancel"),
+          "class": "btn"
+        }];
+
+        buttons.push({
+          "label": Em.String.i18n("composer.reply_original"),
+          "class": "btn-primary",
+          "callback": function(){
+            _this.save(true);
+          }
+        });
+
+        if(topic) {
+          buttons.push({
+            "label": Em.String.i18n("composer.reply_here"),
+            "class": "btn-primary",
+            "callback": function(){
+              composer.set('topic', topic);
+              composer.set('post', null);
+              _this.save(true);
+            }
+          });
+        }
+
+        bootbox.dialog(message, buttons);
+
+        return;
+      }
+    }
+    
     return composer.save({
       imageSizes: this.get('view').imageSizes()
     }).then(function(opts) {

app/assets/javascripts/discourse/models/composer.js

@@ -44,6 +44,10 @@ Discourse.Composer = Discourse.Model.extend({
     return this.get('action') === EDIT;
   }).property('action'),
 
+  replyingToTopic: (function() {
+    return this.get('action') === REPLY;
+  }).property('action'),
+
   viewOpen: (function() {
     return this.get('composeState') === OPEN;
   }).property('composeState'),

app/assets/javascripts/discourse/routes/topic_route.js

@@ -27,6 +27,8 @@ Discourse.TopicRoute = Discourse.Route.extend({
     topicController = this.controllerFor('topic');
     topicController.cancelFilter();
     topicController.set('multiSelect', false);
+    this.controllerFor('composer').set('topic', null);
+
     if (headerController = this.controllerFor('header')) {
       headerController.set('topic', null);
       headerController.set('showExtraInfo', false);
@@ -35,6 +37,7 @@ Discourse.TopicRoute = Discourse.Route.extend({
 
   setupController: function(controller, model) {
     this.controllerFor('header').set('topic', model);
+    this.controllerFor('composer').set('topic', model);
   }
 
 });

config/locales/client.en.yml

@@ -260,6 +260,7 @@ en:
         message: "Authenticating with Persona (make sure pop up blockers are not enabled)"
 
     composer:
+      posting_not_on_topic: "You have composed a reply to \"{{title}}\", however you are no longer looking at that topic."
       saving_draft_tip: "saving"
       saved_draft_tip: "saved"
       saved_local_draft_tip: "saved locally"
@@ -269,7 +270,10 @@ en:
         more: "{{n}} to go..."
 
       save_edit: "Save Edit"
+      reply_original: "Reply on Original Topic"
+      reply_here: "Reply Here"
       reply: "Reply"
+      cancel: "Cancel"
       create_topic: "Create Topic"
       create_pm: "Create Private Message"