include vbox into .cell css definition

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

@@ -109,7 +109,7 var IPython = (function (IPython) {
     CodeCell.prototype.create_element = function () {
         IPython.Cell.prototype.create_element.apply(this, arguments);
 
-        var cell =  $('<div></div>').addClass('cell border-box-sizing code_cell vbox');
+        var cell =  $('<div></div>').addClass('cell border-box-sizing code_cell');
         cell.attr('tabindex','2');
 
         this.celltoolbar = new IPython.CellToolbar(this);

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

@@ -67,7 +67,7 var IPython = (function (IPython) {
      */
     TextCell.prototype.create_element = function () {
         IPython.Cell.prototype.create_element.apply(this, arguments);
-        var cell = $("<div>").addClass('cell text_cell border-box-sizing vbox');
+        var cell = $("<div>").addClass('cell text_cell border-box-sizing');
         cell.attr('tabindex','2');
 
         this.celltoolbar = new IPython.CellToolbar(this);

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

@@ -171,6 +171,7 div.ui-widget-content {
 
 .cell {
     border: 1px solid transparent;
+    .vbox();
 
     &.selected {
         .corner-all;