include hbox in outpuarea css

IPython/frontend/html/notebook/static/js/outputarea.js

@@ -240,7 +240,7 b' var IPython = (function (IPython) {'
 
 
     OutputArea.prototype.create_output_area = function () {
-        var oa = $("<div/>").addClass("hbox output_area");
+        var oa = $("<div/>").addClass("output_area");
         if (this.prompt_area) {
             oa.append($('<div/>').addClass('prompt'));
         }

IPython/frontend/html/notebook/static/less/notebook.less

@@ -273,6 +273,7 b' div.output_prompt {'
 div.output_area {
     padding: 0px;
     page-break-inside: avoid;
+    .hbox();
 }