Under Dyalog 19.0.48959, if I click on the Boxing on/off menu before running any User Command in the OUTPUT group, I get the a value error and in the Status popup box i get the message
I do not get this error under version 18.2⎕SE.cbtop.bandtb5.tb.boxing: Callback '⎕SE.Dyalog.Out.flipBox' is not a defined function