I used to have a console.warn() here because it was unexpected. But then I found it happening routinely when I swipe all the way off the screen.  

I set newtarget back to oldtarget here because I don't think it is valid to fire mouseout and mouseleave events with relatedTarget of null.  And I don't want to have to deal with all the special cases of having a null target.  I think something would break.