Open the Categories window.  Select at least one category.  The contents of the
"Selected Categories" area appears to be a directly editable text entry box.  In
fact, I can highlight a selection of that contents.  It looks as though I should
be able to affect it's contents by typing on my keyboard.  But I can't.  (And I
understand why you would want me to be limited to the defined Available
categories (i.e. How would you handle me making a type?  Add a new category? 
Complain? ).  So, the contents of the "Selected Categories" area should not
appear to be selectable or editible.  Having an overview of which checkboxes
have been selected is quite nice -- please do not remove it -- just change the
UI so that it is clear that it's contents only _reflect_ the choices below.