Changing CodeMirror-scroll to overflow-y: hidden.

* It seems that our old setting of visible was causing scroll bars
  to appear if font sizes change in Chrome.
* Also changed json -> xml in Download UI.

IPython/frontend/html/notebook/static/css/notebook.css

@@ -266,13 +266,15 b' div.text_cell_render {'
 }
 
 .CodeMirror {
-  line-height: 1.231;  /* Changed from 1em to our global default */
+    line-height: 1.231;  /* Changed from 1em to our global default */
 }
 
 .CodeMirror-scroll {
-  height: auto;     /* Changed to auto to autogrow */
-  overflow-y: visible; /* Changed from auto to remove scrollbar */
-  overflow-x: auto; /* Changed from auto to remove scrollbar */
+    height: auto;     /* Changed to auto to autogrow */
+    /*  The CodeMirror docs are a bit fuzzy on if overflow-y should be hidden or visible.*/
+    /*  We have found that if it is visible, vertical scrollbars appear with font size changes.*/
+    overflow-y: hidden;
+    overflow-x: auto; /* Changed from auto to remove scrollbar */
 }
 
 /* CSS font colors for translated ANSI colors. */

IPython/frontend/html/notebook/templates/notebook.html

@@ -62,7 +62,7 b''
                 <div class="section_row">
                     <span>
                         <select id="download_format">
-                            <option value="json">json</option>
+                            <option value="json">ipynb</option>
                             <option value="py">py</option>
                         </select>
                     </span>