I see that danob suggested,

click the clear button on the options bar and the tool will behave normally again.

Thanks for the tip, but that wasn't the cause ... I had not ever used those options ... and nothing was preset there for the crop tool. 

Just in case it would have an effect (or because there was some clever, covert, secret, hidden, occult, or ... setting), I did try the clear button.  The cursor still had that ~8 px 'forbidden' zone. 

Ahhhhhhhhhh ... the mysteries of Photoshop ...

Still hoping for ...

--
Martin