core-service.js

  /**
   * Set N latest non visible messages to visible.
   */
  revealLastNmessages: function() {
    let hiddenMessages = this.get('store').peekAll('chat-message').filterBy('isVisible', false);
    let l = hiddenMessages.get('length');

    // Show latest N messages.
    hiddenMessages.slice(l - MAX_MESSAGES_COUNT, l)
                  .setEach('isVisible', true);
  },

router.js

    /**
     * User scrolled up to the top of the messages list.
     * Lets reveal more messages from history.
     */
    scrolledToTop() {
      let messagesElmt = Ember.$('.messages-container')[0];
      let originalScrollHeight = messagesElmt.scrollHeight;

      this.get('core').revealLastNmessages();

      // Once our elements are added lets scroll
      // back to the previous position.
      run.scheduleOnce('afterRender', this, function() {
        Ember.$('.messages-container').scrollTop(messagesElmt.scrollHeight - originalScrollHeight);
      });
    },