the content that's presently selected. If provided
the height of our page this provides the scroll position as well...

::

  selected, scroll = my_scroller.selection(content, page_height)

:param list content: content the scroller is tracking
:param int page_height: height visible on the page

:returns: **tuple** of the form **(cursor, scroll)**, the cursor is
  **None** if content is empty
r