The keyboard navigation in drop down lists is currently inconsistent based on the widget configuration.  When not filterable, the drop down allows you to type the first letter of one of the options in order to select it when the drop down is closed.  For example, tab to the drop down, do not open it, and type 'b' to select Bob. 

When the drop down is filterable, that behavior does not work.  You must use alt-down to expand the drop down and then begin filtering.  

I propose that the widget can consistently handle both cases by detecting whether the drop down is currently open in the keydown handler.  If filterable, and open, move into the filtering logic.  If filterable, and closed, stick with the standard selection logic.