I tested it at chrome 48.0.2564.109.
<!-- Create a simple CodeMirror instance -->
I'am sorry, My English is poor.
The demo at http://codemirror.net does not show a scrollbar when there's only a single line in it, neither could I scroll it with the scroll wheel. What kind of scrolling do you mean?
It's not scrollbar.
When CodeMirror has only a single line, The text in CodeMirror will up and down when the mouse are scrolling
Remove accidental gap above CodeMirror-gutter elements
Ah, now I can see it. Attached patch fixes it for me (on Chrome)