Fix the prompt_area argument of the output area constructor.

See #2681 (comment) and jupyter-widgets/ipywidgets#1765

Author: jasongrout

notebook/static/notebook/js/outputarea.js

@@ -77,6 +77,9 @@ define([
 
     OutputArea.prototype.style = function () {
         this.collapse_button.hide();
+        if (!this.prompt_area) {
+            this.prompt_overlay.hide();
+        }
 
         this.wrapper.addClass('output_wrapper');
         this.element.addClass('output');