Don't disable the composer's submit button. Let it be clicked to cause validation error messages to show.

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

@@ -94,6 +94,10 @@ export default Discourse.Controller.extend({
     return false;
   },
 
+  disableSubmit: function() {
+    return this.get('model.loading');
+  }.property('model.loading'),
+
   save: function(force) {
     var composer = this.get('model'),
         self = this;

app/assets/javascripts/discourse/templates/composer.js.handlebars

@@ -84,7 +84,7 @@
 
         {{#if currentUser}}
           <div class='submit-panel'>
-            <button {{action save}} tabindex="4" {{bind-attr class=":btn :btn-primary :create model.cantSubmitPost:disabled"}} title="{{i18n composer.title}}">{{model.saveText}}</button>
+            <button {{action save}} tabindex="4" {{bind-attr class=":btn :btn-primary :create disableSubmit:disabled"}} title="{{i18n composer.title}}">{{model.saveText}}</button>
             <a href='#' {{action cancel}} class='cancel' tabindex="4">{{i18n cancel}}</a>
           </div>
         {{/if}}