FEATURE: if there is only one category, don't show the category picker

app/assets/javascripts/discourse/models/composer.js.es6

@@ -67,6 +67,12 @@ const Composer = RestModel.extend({
   creatingPrivateMessage: Em.computed.equal('action', PRIVATE_MESSAGE),
   notCreatingPrivateMessage: Em.computed.not('creatingPrivateMessage'),
 
+  showCategoryChooser: function(){
+    const manyCategories = Discourse.Category.list().length > 1;
+    const hasOptions = this.get('archetype.hasOptions');
+    return !this.get('privateMessage') && (hasOptions || manyCategories);
+  }.property('privateMessage'),
+
   privateMessage: function(){
     return this.get('creatingPrivateMessage') || this.get('topic.archetype') === 'private_message';
   }.property('creatingPrivateMessage', 'topic'),
@@ -434,6 +440,13 @@ const Composer = RestModel.extend({
     // We set the category id separately for topic templates on opening of composer
     this.set('categoryId', opts.categoryId || this.get('topic.category.id'));
 
+    if (!this.get('categoryId') && this.get('creatingTopic')) {
+      const categories = Discourse.Category.list();
+      if (categories.length === 1) {
+        this.set('categoryId', categories[0].get('id'));
+      }
+    }
+
     if (opts.postId) {
       this.set('loading', true);
       this.store.find('post', opts.postId).then(function(post) {

app/assets/javascripts/discourse/templates/composer.hbs

@@ -48,7 +48,7 @@
                 {{popup-input-tip validation=view.titleValidation shownAt=view.showTitleTip}}
               </div>
 
-              {{#unless model.privateMessage}}
+              {{#if model.showCategoryChooser}}
                 <div class="category-input">
                   {{category-chooser valueAttribute="id" value=model.categoryId scopedCategoryId=scopedCategoryId tabindex="3"}}
                   {{popup-input-tip validation=view.categoryValidation shownAt=view.showCategoryTip}}
@@ -57,7 +57,7 @@
                   <button class='btn' {{action "showOptions"}}>{{i18n 'topic.options'}}</button>
                 {{/if}}
                 {{render "additional-composer-buttons" model}}
-              {{/unless}}
+              {{/if}}
             </div>
           {{/if}}