Prevent a user from leaving a grid line with validation errors

[Michael Adams]

I have an action based screen (no object), with a grid that the user needs to enter details into.

I need to validate that the user hasn't entered duplicate lines into the details grid.
The grid has multiple fields that make up the unique key, so I can't do an after change / control on a specific field.

I  would like to validate the line after the user tabs off to the next line or clicks a button, which I can do in the NBLIG Control
action by trapping status = 75.  However, setting mkstat = 2 doesn't prevent X3 from moving to the next grid line.

How can I prevent X3 moving to a new grid line if there are validation errors ?

e.g.

Subprog C_NBLIG
  If (status = 75) Then
...
    if (validation errors) then
      mkstat = 2  # Doesn't do anything
      end
   endif

...

Regards,
Michael