Fixed modal centering code

IPython/html/static/notebook/js/widgets/container.js

@@ -125,11 +125,13 define(["notebook/js/widget"], function(widget_manager) {
         },
         
         show: function() {
-            this.$window.show();
             this.$show_button.addClass('btn-info');
-            this.$window.css("top", Math.max(0, (($(window).height() - this.$window.outerHeight()) / 2) + 
+
+            this.$window.show();
+            this.$window.css("positon", "absolute")
+            this.$window.css("top", Math.max(0, (($('body').outerHeight() - this.$window.outerHeight()) / 2) + 
                                                 $(window).scrollTop()) + "px");
-            this.$window.css("left", Math.max(0, (($(window).width() - this.$window.outerWidth()) / 2) + 
+            this.$window.css("left", Math.max(0, (($('body').outerWidth() - this.$window.outerWidth()) / 2) + 
                                                         $(window).scrollLeft()) + "px");
         },