Be a little more clear about sizing logic.

IPython/html/static/edit/js/main.js

@@ -1,90 +1,97 b''
 // Copyright (c) IPython Development Team.
 // Distributed under the terms of the Modified BSD License.
 
 require([
     'jquery',
     'base/js/namespace',
     'base/js/utils',
     'base/js/page',
     'base/js/events',
     'contents',
     'services/config',
     'edit/js/editor',
     'edit/js/menubar',
     'edit/js/savewidget',
     'edit/js/notificationarea',
     'custom/custom',
 ], function(
     $,
     IPython,
     utils,
     page,
     events,
     contents,
     configmod,
     editmod,
     menubar,
     savewidget,
     notificationarea
     ){
     page = new page.Page();
 
     var base_url = utils.get_body_data('baseUrl');
     var file_path = utils.get_body_data('filePath');
     contents = new contents.Contents({base_url: base_url});
     var config = new configmod.ConfigSection('edit', {base_url: base_url});
     config.load();
     var common_config = new configmod.ConfigSection('common', {base_url: base_url});
     common_config.load();
     
     var editor = new editmod.Editor('#texteditor-container', {
         base_url: base_url,
         events: events,
         contents: contents,
         file_path: file_path,
         config: config,
     });
     
     // Make it available for debugging
     IPython.editor = editor;
     
     var save_widget = new savewidget.SaveWidget('span#save_widget', {
         editor: editor,
         events: events,
     });
     
     var menus = new menubar.MenuBar('#menubar', {
         base_url: base_url,
         editor: editor,
         events: events,
         save_widget: save_widget,
     });
     
     var notification_area = new notificationarea.EditorNotificationArea(
         '#notification_area', {
         events: events,
     });
     notification_area.init_notification_widgets();
 
     utils.load_extensions_from_config(config);
     utils.load_extensions_from_config(common_config);
     editor.load();
     page.show();
 
     window.onbeforeunload = function () {
         if (editor.save_enabled && !editor.codemirror.isClean(editor.generation)) {
             return "Unsaved changes will be lost. Close anyway?";
         }
     };
 
     // Make sure the codemirror editor is sized appropriatley.
     var _handle_resize = function() {
         var header = $('#header');
-        var padding = header.outerHeight(true) - header.innerHeight();
-        $('div.CodeMirror').height(window.innerHeight - header.height() - 2*padding);
+
+        // The header doesn't have a margin or padding above it.  Calculate
+        // the lower margin&padding by subtracting the innerHeight from the
+        // outerHeight.
+        var header_margin_bottom = header.outerHeight(true) - header.innerHeight();
+
+        // When scaling CodeMirror, subtract the header lower margin from the
+        // height twice.  Once for top padding and once for bottom padding.
+        $('div.CodeMirror').height(window.innerHeight - header.height() - 2*header_margin_bottom);
     };
     window.onresize = _handle_resize;
 
     // On document ready, resize codemirror.
     $(_handle_resize);
 });