How can I invoke the Large Data Editor for a VARCHAR2 column with the length < 20 characters?

I would like to see the cell value in HEX!

SQL Query
select cast('A' || chr(9) || 'B' as varchar2(20)) from dual;
This is OK, a mouse click on cell button and then Hex-Tab.


But...
SQL Query
select cast('A' || chr(9) || 'B' as varchar2(19)) from dual;
How can I see the cell value in HEX in this case?