Set body's visibility to hidden when unloading

Previously the display was changed to none via $.hide() which accidentally
affected the ability of package's to serialize DOM properties such as
scrollTop since the value would always be zero when the display was none.

The goal here is to just prevent a flicker when refreshing the
editor window and setting visibility to hidden still accomplishes this.

src/atom.coffee

@@ -194,7 +194,7 @@ class Atom
     @menu.update()
 
     $(window).on 'unload', =>
-      $(document.body).hide()
+      $(document.body).css('visibility', 'hidden')
       @unloadEditorWindow()
       false